Warning!
You want Steam to be shutdown, once finish work with appmanifest file, make a copy just incase, you can launch Steam to see if you did it correctly, if didn't do it right, review your work.

You need to know your game ID, to find out about it, need to either visit game store page, read URL address for it as the numbers is the game ID, or if game delisted, and no longer has a store page, can use Steamdb.info that show you how to find game ID. Each game has their own ID number, that why need to know what it is, as this is something you're gonna need for Step 2.
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For game example we're gonna be using Vampire Survivors, as it only thing ATM I have that pending for update.

Once you figure out what your Game ID number is, move on to step 2.

You want to head to Steam folder where your game is installed, but not looking for game folder, you're looking for folder call "steamapps" this holds key file you need to edit, this is what tricks Steam client thinking game is updated.
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If you have the game installed on another drive, instead of the default Steam folder, you're still looking for "steamapps" folder but it be on the other drive that the game is on.
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When you're in the Steamapps folder, you're looking for the said game file you need to edit it be "appmanifest_#######.acf" <--- replace # = game id number, so in this case example we're looking for Vampire Survivors file which is "appmanifest_1794680.acf" notice the numbers matching Game ID that why need game ID from Step 1.
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You know for sure if it the right file when you open it with text editor, and read the name, and ID.

Once you found your appmanifest file, move onto step 3.

Now this is where the hard work comes into play, but it annoying if game has bunch of DLCs that you have installed for it. We're only changing number of things, the rest of the work for you to do as all games has their own ID's that informs Steam client, if get lost follow the screenshot as an example to try understand what going on, but do not copy things you see from this guide as they're not the same for another game, or if game got update as this is only for this game Vampire Survivors ATM of typing this out, it id will change later when another update happens.

So you have the game appmanifest open, as can see below.


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Visit https://steamdb.info/ you be using this great site to find the game history page, you need to see what the latest build ID, and it changes for the game, without it you won't be able to trick client into thinking it updated.
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So you're now on game historty page, to match things to what they are it simply copy, and paste, a train monkey can do it *insert monkey noises*, now what you're doing is important facts need to know, if you opt your game beta, you need to look what the branch is called, otherwise normal game version is called "public" as can see from Orange arrow in screenshow, for green arrow that your depot, and red arrow it pointing is your manifest changes need to do.
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For starters we need to to know what is build ID changed to, you can look on steamdb.info visit game history page, press Ctrl + F to bring up search bar, type in buildid, look for the latest one that be the 1st one, and want to copy the green number as you see as that the latest version of game build ID is, and replace the build ID you have in game file, now if have game too far out of date, you need make sure targetbuildid is same as buildid both need to be the same numbers. Once you done that, you only need to change two things before getting into hardest part. So change "stateflags" number that say 6 to 4, and for "scheduledautoupdate" change to 0 <--- zero that it.
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Now You're looking to match whatever ID for depot that is display from your game file on the site the number is always below the list of text column, you see if you have more depot if you look in the same column position from two screenshots below to know how to find depot in game file, as the game we're using only has one depot, the other screenshot has more than one depot because it has DLCs, and addons. The red arrows pointing at the numbers those are depot, the red box is the section of the depot, what want to look for it depot ID.

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Now you know how to find your depot, you want to copy your depot number, and visit the game history page on steamdb.info site, press ctrl + F to bring up search, and paste in the number, this will highlight all the numbers matching it, once found go the the latest one at the top, and ensure it say public if your using public build of the game = normal game version. See screenshot of green arrow pointing to depot to depot see how they match. It's that easy to finding it. Red arrow pointing to show you the latest changes, so see red numbers, that is outdated, you want to copy the green numbers, and replace the numbers that red arrow pointing to that it, you finished updating your depot. Make sure you don't make a mistake.
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Here are the changes see how I made the changes like so from old to new, look at this screenshot, and one above to see the arrows pointing at the depot, and changes needed to be done it not hard at all, just take a bit to get used to, and more work if there more depots you need to update in your file.
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As you can see when I launch the game, the game hasn't updated, and able to play the game just fine.
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This is what the game look like when it is updated.

Now if you're still confused how to do number updates, and wonder why it not working, IF YOU HAVE DLC's you ALSO have to update their numbers as well in the same game appmanifest file please note you have to do this for ALL dlc if there has been a change to them. You have learn how to look for game ID, in the appmanifest it likely list the DLC ID as well, but if it doesn't you want to search up your game DLC on Steamdb, and check on history for it, from there it's same deal, look for it depot, find it latest update number, and just copy & paste that it easy stuff.

Here screenshots to explain things better please read carefully.

Here is answers to questions, if have a question that worth noting will be added here.

Q: I'm confused where do I find my game installed.
A: Right click on game from steam library > properties > local files > Browse. Now you know where game installed.

Q: Why do I have to do all this work?
A: To prevent it from updating... Duh...

Q: How often do I have to do this?
A: Whenever an update is pushed, or if Steam client think there's an update need to be done.

Q: How long will it take to do this?
A: Depends if you're able to read, not cherry pick what you want to read, and pay attention what you're doing.

Q: Is there not a better way?
A: Not ATM unless Steam gives us skip update option, this is all self work, no downloads, no extra software.

Q: I have a ton of DLC's, and seem there a lot of depots I have to update.
A: Well get to it, because that what you're gonna make sure they're upto date with latest.

Q: Do DLC's get updates?
A: Depends if game dev push an update that affects DLC, and it depot, if not then there nothing to update for it as long it already latest version. That why compare, and see if there changes, if see none, there don't need to do anything for it.

Q: What if I want to keep my game version, and download DLC?
A: You're updating at that point, might as well just update, or figure out how you're gonna get the right version of the DLC for your game version, and manually add Depot into your game appmanifest file.

Q: What if I want to remove DLC?
A: Well you're updating then, unless you can figure out how to manually remove DLC, if you did, then just remove the depot from the appmanifest file as well, that it.

Q: What if I want the latest update for DLC, but keep older game version?
A: Well have fun if something breaks, or something went wrong, as sometimes games may freak out, or not, or just stop working, or may even ruin your save, so it your own risk what you want to do. I would say don't bother, and just update if want latest update of DLC.

Q: I have mods installed, does that matter?
A: If this is not Steamworkshop mods then no it doesn't matter, but if it is Steamworkshop mods, well you're gonna update your game, or have fun hunting down what need to figure out what changes need to be done in appmanifest file for it, as well figure out if it not gonna break, or something, mods are weird sometimes.

Q: Can I do this for multiplayer games.
A: Yes, but don't do it for that reason, there risk may face consequences such as getting ban by system, or game not working, so just update, no reason not update if this is an online game, or multiplayer game, save yourself pain by just doing easy way out, which is updating.

Q: Will you help me wit...
A: NO! Do your own work, all info is in the guide, just read, review screenshots. People got their own lives, and don't want to burn away their time for nothing because didn't read.

Q: Will this guide fix mods?
A: No, has nothing to do with mods, it's to prevent game from updating, that it.

Q: Will you make an app to do work for me?
A: No, just pray someone else might make an app to do this kind of work in the future.

Q: Will this work for software, and not just games?
A: Yes, same thing, but unless app has some kind of file checker of it own to connect to server, or something then might be out of luck.

Q: I manage to downgrade my game version will this guide work?
A: Yes it tricks steam client thinking your game is installed, that all it does. So if somehow downgraded your game, you be able to play it.

Q: If I verify my game files, will it update?
A: Yes because it file checked, that why you don't verify game file.

Q: I started download process does this revert game downloaded, or parted patched already?
A: No that not how this guide works, it to prevent updating, not revent, or you from already downloading, don't click update, or verify game file, or else it will update.

Q: So I understood the things I need to change, do I just save?
A: Yes, you save, once done, you can then launch steam, and see if it worked.