To install mods you need mod loader called "HedgeModManager".
with this tool it's possible to install likely any Sonic Frontiers (and other Sonic games) Mods, also it lets you customize game settings and preferences.

• First, download HMM[gamebanana.com]. Mention that it's better to get the latest version of this tool.
  
• Open the HedgeModManager.exe (if you get warning from Windows / Mac just ignore it).
  
• Launch the HMM and in Mod Directory just add the location of the game (it should be like that: "SteamLibrary\steamapps\common\SonicFrontiers" or so).
  
• Mention that some mods will likely need different location or methods to install them.
  
• Note: You need to use the program 'Hedgemodmanager' in order to make the mods work. And switch Update Channel setting to 'Development' in the settings. (thanks to ChōjinColor)

HD Characters[gamebanana.com]
this mod overhaul and optimize textures and characters to 4k resoluation.
currently the mod is not quite stable but still worth trying.
Credit: spoonScribble[gamebanana.com]


Changed Main Menu Background[gamebanana.com]
Changes the Main Menu to official Art.
Credit: StarNights/Siouxsie[gamebanana.com]


Better Cyber Space Stage Names[gamebanana.com]
Adds names to all Cyber Space stages.
Credit: GeekCritique[gamebanana.com]


Improved Arcade UI[gamebanana.com]
Adds level names to the banner of the levels in Arcade Mode. Optionally, you can enable a descriptor which shows what dimension the level uses! These can appear on the banner, in the level title, both or neither. This can be customised from within HedgeModManager.
Credit: keanine[gamebanana.com]

Among Us Crewmates Over Koco[gamebanana.com]
This mod replaces the base Koco models with the red crewmate from Among Us.
Credit: CyclonX[gamebanana.com]


Sonic 06[gamebanana.com]
a mod that adds sonic skin from sonic the hedgehog 2006
Credit: Taku/Wailful[gamebanana.com]


Super Sonic Frontiers[gamebanana.com]
Credit: Anferny[gamebanana.com]


Pac-Man Overhaul[gamebanana.com]
Pac man skin model from Sonic Dash
Credit: Jack-Rabbit[gamebanana.com]


Silver the Hedgehog[gamebanana.com]
This mod replaces Sonic with Silver the Hedgehog
Credit: AGhostsPumpkinSoup[gamebanana.com]


Mario (Mario Kart 8 Model)[gamebanana.com]
Credit: SebGoP222[gamebanana.com]


Scourge The Hedgehog[gamebanana.com]
that mod add Scourge The Hedgehog skin
Credit: Davihhh[gamebanana.com]


Rush Adventure Sonic[gamebanana.com]
Amazing Model from Sonic Rush Adventure now in Sonic Frontiers!
Credit: SmileyWorld[gamebanana.com]


Tails Frontiers[gamebanana.com]
This mod replaces Sonic's model with Tails seen from Peter Knetter's recent video!
Credit: Bluwolfblitz[gamebanana.com]

Alternate Main Menu[gamebanana.com]
This mod replaces the main menu background with the Rail Grinding Art from pre release / steam.
Credit: Sticks Unleashed[gamebanana.com]


Hill Top Theme Over Green Hill[gamebanana.com]
mod that changing the rocks and some other textures in Hill Top Zone.
Credit: TheHooder2002[gamebanana.com]


PS2 Unleashed Ranking Icons[gamebanana.com]
changes the ranking icons to PS2 & Wii Unleashed icons.
Credit: TheHooder2002[gamebanana.com]


Snowy Ares Island[gamebanana.com]
add snowy mountains
Credit: 7yu7[gamebanana.com]


HD Sonic[gamebanana.com]
This mod gives both Sonic and Super Sonic custom-made 4K texturesets for his fur, cloth, and skin, along with smoother and higher poly models (including aura).
Credit: ?

Fall Foliage on Kronos Island[gamebanana.com]
give fall colors for the game.
Created by pupsi[gamebanana.com]


Potato Low End Foliage[gamebanana.com]
mod that ported from Nintendo Switch and should improve FPS on Low-End PCs.
Created by WaferKat[gamebanana.com]

here you will find mods that should release in the future, i just choose some of my favorites.

Proper Ultrawide
mod that will soon be available and should add option for Ultrawide Resolution for monitors that support it.


Fall Guys sonic skin mod[gamebanana.com]
a nice sonic skin from fall guys.

more upcoming mods can be found here: https://gamebanana.com/requests/games/15779

Updated 20/11/2022
Added:
Changes the Main Menu to official Art in Graphics
Better Cyber Space Stage Names in Graphics
Sonic 06 in Characters
Super Sonic Frontiers in Characters
Pac-Man Overhaul in Characters
Silver the Hedgehog in Characters
Mario (Mario Kart 8 Model) in Characters
PS2 Unleashed Ranking Icons in Textures
========================================
Updated 12/12/2022
Added:
Snowy Ares Island in Textures
HD Sonic in Textures
Scourge The Hedgehog in Characters
Rush Adventure Sonic in Characters
Tails Frontiers in Characters
Improved Arcade UI in Graphics
important note in Installation section

if you have any issues or problems with the mods, you can always get help from:
Sonic Retro[info.sonicretro.org]
Gamebanana[gamebanana.com]
Hedgehog Engine Modding Discord[discord.com]
Sonic News Network[sonic.fandom.com]

and also feel free to comment here.