To upgrade from standart version you firt need to own Weapon Workshop
What is a weapons workshop ? - its basically a workshop where you upgrade from standart to mk2 variant.

Where do i find/purchase Weapon Workshop ? - By purchasing the MOC - Mobile Operation Centre or the Terrorbyte or Avenger you should see an option with Weapon Workshop , just select it and there you go ! Or by purchasing the Arcade and starting the Casino heist you should see your GunMan by his workbench and also having the Weapon Workshop .

The answer is money...

Pistol Mk II - $73,750
SMG Mk II - $85,500
Assault Rifle Mk II - $98,750
Carbine Rifle Mk II - $107,500
Combat MG Mk II - $119,000
Heavy Sniper Mk II - $165,375

There are also other things you can put on your guns such as :

Tracer Rounds, which show bright visible markers matching the weapon tint
Incendiary Rounds, which have a chance to set targets on fire
Hollow Point Rounds, which increase damage to unarmored targets
Armor Piercing Rounds, which increase penetration of Body Armor
Full Metal Jacket Rounds, which increase damage to vehicles and penetrate bullet proof and bullet resistant glass
Explosive Rounds, which create a small explosion on impact
Over 80 new weapon attachments, liveries and tints have been added for Mk II weapons, some of which are unlocked via Research. These include:
A selection of different scopes for each Mk II weapon, including Night Vision and Thermal vision scopes for the Heavy Sniper Mk II
Heavy Barrels, which increases damage dealt to long-range targets
Muzzle Brakes and Compensators, which reduce recoil during rapid fire
Attaching weapons to several vehicles.
New camos and paintjobs.

I have already mention this in previous section but ill whrite it down here one more time

Pistol Mk II - $73,750
SMG Mk II - $85,500
Assault Rifle Mk II - $98,750
Carbine Rifle Mk II - $107,500
Combat MG Mk II - $119,000
Heavy Sniper Mk II - $165,375

Im not sure if they added more weapons by future if yes let me know down in the comment section :D

- They do more damage than the standart weapons
- For exam. the Heavy Sniper MK2 features explosive bullets which can be extra usefull in some missions
- Over 80 new weapon attachments, liveries and tints have been added for Mk II weapons, some of which are unlocked via Research
-

Performance
Grand Theft Auto Online
The Heavy Sniper Mk II improves over the regular Heavy Sniper in damage (from 216 to 230), with the same rate of fire. Despite having the same damage multiplier against other players (x1.39) as the regular Heavy Sniper, one shot with regular ammo in the chest is still not enough to kill a high level player at full health. However, they can still cause severe damage and retains its damage output against vehicles, especially aircraft.

Along with the standard Zoom Scope and the Advanced Scope, the weapon also has access to Night Vision and Thermal Scopes; despite having lesser zoom level than the Advanced Scope, they allow to spot players on dark areas or detect heat signatures. It also has access to an Extended Clip, increasing magazine capacity to 8 rounds, as well as a Suppressor intended for stealth attacks (making it the most powerful suppressed weapon). Muzzle Brakes are also available to reduce recoil, although it is not very noticeable due to the low fire rate.

The weapon has access to the following ammo types:

Incendiary rounds, which have chances of setting enemies on fire. Due to the low fire rate and small capacity, incendiary rounds will guarantee to ignite targets upon contact.
Armor Piercing rounds, which increase damage over armored targets. This can help at taking out targets from the body easier, as Body Armor may decrease the overall effectiveness of the weapon.
Full Metal Jacket rounds, which improve damage over vehicles and can penetrate bullet-resistant windows. This can be useful against unarmored vehicles due to the high damage.
Explosive rounds, which give a destructive firepower against unarmored vehicles. A single shot can destroy these instantly depending on the type.
Note that specialized ammo magazines have a reduced capacity of 4 rounds, with an overall capacity of 80 rounds (20 magazines). Explosive ammo have a further reduced capacity of 40 rounds (10 magazines).

Performance
Grand Theft Auto Online
The Special Carbine Mk II slightly improves over the regular Special Carbine in damage (from 32 to 32.5), with the same rate of fire. Compared to other assault rifles, it has the second highest damage per second, only beaten out by the Bullpup Rifle Mk II but still outperforms the Carbine Rifle Mk II and Assault Rifle Mk II. (The latter of which has an elevated damage model of 40 but is offset by having the lowest rate of fire).

The damage per second does not rival the Combat MG Mk II, but like most of the other assault rifles, it retains stellar accuracy and minimal spread during fully automatic fire, particularly when equipped with attachments such as the grip, meaning that it will become a tradeoff between damage or accuracy.

The weapon has access to the following ammo types:

Tracer rounds, which only have an aesthetic effect. A tracer round is shot every three bullets, with the last three being also tracer rounds. Overall capacity is 360 rounds (12 magazines).
Incendiary rounds, which have moderate chances of setting enemies on fire.
Armor Piercing rounds, which increase damage over armored targets. This helps at taking out armored enemies easily.
Full Metal Jacket rounds, which improve damage over vehicles and can penetrate bullet-resistant windows. This helps at taking out targets inside armored vehicles at medium ranges.
Note that specialized ammo magazines have a reduced capacity of 20 rounds, with an overall capacity of 240 rounds (12 magazines).

Performance
Grand Theft Auto Online
The Carbine Rifle Mk II slightly improves over the regular Carbine Rifle in terms of damage (from 30 to 33), with the same rate of fire. This makes the gun have a raw firepower 6% under that of the Special Carbine. Despite seeming like an insignificant upgrade, the 1 extra damage per shot means that one fewer shot to the chest is required to kill a high-level player.

Overall, the Carbine Rifle Mk II gains a noticeable improvement in terms of accuracy and reduced spread, even during fully automatic fire. When equipped with attachments such as a scope, the Carbine Rifle Mk II can put down extremely accurate fire on target with barely any deviation from spread.

The weapon has access to the following ammo types:

Tracer rounds, which only have an aesthetic effect. A tracer round is shot every three bullets, with the last three being also tracer rounds. Overall capacity is 360 rounds (12 magazines).
Incendiary rounds, which have moderate chances of setting enemies on fire.
Armor Piercing rounds, which increase damage over armored targets. This helps at taking out armored enemies easily.
Full Metal Jacket rounds, which improve damage over vehicles and can penetrate bullet-resistant windows. This helps at taking out targets inside armored vehicles at medium ranges.
Note that specialized ammo magazines have a reduced capacity of 20 rounds, with an overall capacity of 240 rounds (12 magazines).

The weapon appears to combine aspects from both the Mark 48 and the M60E4 (also known as Mark 43). The receiver and trigger assembly are similar to that of the Mk 48, while the barrel assembly are similar to those of the Mk 43, particularly the front sight mounted over the barrel muzzle. The handguard appears to be loosely based on that of the L86 LSW, except that it encloses the barrel assembly and features a lower picatinny rail. Like the Mk 48, the Combat MG Mk II features a small heat shield with a small rail on top. Rather than a solid stock, the weapon employs a lightweight skeletal stock.

Performance
Grand Theft Auto Online
The Combat MG Mk II slightly improves over the regular Combat MG in damage (from 45 to 47), with the same rate of fire. This makes it a devastating weapon against enemies and unarmored vehicles.

The weapon has access to the following ammo types:

Tracer rounds, which only have an aesthetic effect. A tracer round is shot every five bullets, with the last ten being also tracer rounds. Overall capacity is 600 rounds (6 magazines).
Incendiary rounds, which have chances of setting enemies on fire, although lower in comparison to other weapons due to the large capacity and high fire rate.
Armor Piercing rounds, which increase damage over armored targets. This helps at taking out armored enemies easily, again due to the large capacity and fire rate.
Full Metal Jacket rounds, which improve damage over vehicles and can penetrate bullet-resistant windows. This can be useful against unarmored vehicles due to the capacity and fire rate, and can be a good choice against standard aircraft, as they are cannot sustain heavy gunfire.
Note that specialized ammo magazines have a reduced capacity of 80 rounds, with an overall capacity of 480 rounds (6 magazines).

Performance
Grand Theft Auto Online
The Pistol Mk II improves over the regular Pistol in damage (from 26 to 32), with the same rate of fire. While a significant upgrade, the Pistol Mk II has average damage per second for a semi-automatic pistol, and is still nowhere near as powerful as many of its counterparts.

The weapon has access to the following ammo types:

Tracer rounds, which only have an aesthetic effect. A tracer round is shot every three bullets, with the last three being also tracer rounds. Overall capacity is 360 rounds (30 magazines).
Incendiary rounds, which has higher chances of setting enemies on fire, though not 100%.
Hollow Point rounds, which increase damage over unarmored targets. This can be useful when having a Suppressor attached, as it counters the damage reduction from the aforementioned attachment.
Full Metal Jacket rounds, which improve damage over vehicles and can penetrate bullet-resistant windows. However, its rate of fire limits its potential against armored vehicles.
Note that specialized ammo magazines have a reduced capacity of 8 rounds, with an overall capacity of 240 rounds (30 magazines).

Performance
Grand Theft Auto Online
The Assault Rifle Mk II improves over the regular Assault Rifle in damage (from 30 to 40), with the same rate of fire. Despite its sluggish rate of fire, the Assault Rifle Mk II's damage per second manages to be slightly superior to the Carbine Rifle Mk II. Adding to that, its high damage per shot makes it a good choice to use specialized ammo.


The weapon has access to the following ammo types:

Tracer rounds, which only have an aesthetic effect. A tracer round is shot every three bullets, with the last three being also tracer rounds. Overall capacity is 360 rounds (12 magazines).
Incendiary rounds, which have moderate chances of setting enemies on fire.
Armor Piercing rounds, which increase damage over armored targets. This helps at taking out armored enemies easily.
Full Metal Jacket rounds, which improve damage over vehicles and can penetrate bullet-resistant windows. This helps at taking out targets inside armored vehicles at medium ranges.
Note that specialized ammo magazines have a reduced capacity of 20 rounds, with an overall capacity of 240 rounds (12 magazines).

The entirety or the gun keeps the general design layout of its base model, but receives a squared and bulkier forend, different vent details, square-shaped trigger guard and a different pattern on the trigger grip. The cylinder takes strong similarities with that of the Chiappa Rhino and the upper side features an accessory rail that allows for scope attachments.

Performance
Grand Theft Auto Online
Compared to the default Heavy Revolver, it deals +25% damage per shot (200 instead of 160) and shoots almost 50% faster (due to it being used in double-action mode as opposed to single-action), making its damage output almost doubled. It retains the +60% damage bonus against vehicles that the regular Heavy Revolver had. Unlike the default Heavy Revolver, its damage per shot is affected by range, starting to fall 20m away to reach 70% at 120m. As a result, it is capable of destroying the rear rotor blades of an helicopter in one single shot at close range.

The weapon has access to the following ammo types:

Tracer rounds, which only have an aesthetic effect. All bullets are tracer rounds. Overall capacity is 360 rounds.
Incendiary rounds, which have chances of setting enemies on fire. Due to the low fire rate, incendiary rounds will guarantee to ignite targets upon contact.
Hollow Point rounds, which increase damage over unarmored targets. Since the weapon is already powerful, one shot in the chest is enough to kill a player.
Full Metal Jacket rounds, which improve damage over vehicles and can penetrate bullet-resistant windows. Due to its increased damage against vehicles, FMJ rounds can destroy an helicopter's tail rotor at any range or explodes a car in the oil tank in a single shot.
Note that specialized ammo types retain the standard capacity of 6 rounds, although overall capacity is reduced to 240 rounds.

The weapon has a more squared-off appearance, along with a thinner barrel and straighter carry handle. The forend now employs a single picatinny rail on the right side and the stock is slightly larger.

Performance
Grand Theft Auto Online
The Bullpup Rifle Mk II slightly improves over the regular Bullpup Rifle in damage (from 32 to 33), with the same rate of fire as the Mk I. When compared to other assault rifles, it has the highest damage per second, even surpassing the Special Carbine Mk II, due to the increased rate of fire despite sharing a similar damage profile of 32 and 33.

The damage per second does not rival the Combat MG Mk II, but like most of the other assault rifles, it retains stellar accuracy and minimal spread during fully automatic fire, particularly when equipped with attachments such as the grip, meaning that it will become a tradeoff between damage or accuracy.

The weapon has access to the following ammo types:

Tracer rounds, which only have an aesthetic effect. A tracer round is shot every three bullets, with the last three being also tracer rounds. Overall capacity is 360 rounds (12 magazines).
Incendiary rounds, which have moderate chances of setting enemies on fire.
Armor Piercing rounds, which increase damage over armored targets. This helps at taking out armored enemies easily.
Full Metal Jacket rounds, which improve damage over vehicles and can penetrate bullet-resistant windows. This helps at taking out targets inside armored vehicles at medium ranges.
Note that specialized ammo magazines have a reduced capacity of 20 rounds, with an overall capacity of 240 rounds (12 magazines).

I mainly got the informations from Gta Wiki [gta.fandom.com]