https://www.reddit.com/r/Steam/comments/611h5 … s_of_a_game_on/

Since it appears that my post yesterday about the command to download older game files was useful, I thought I'd write a full gu […]
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Since it appears that my post yesterday about the command to download older game files was useful, I thought I'd write a full guide on how to find older versions of games and download them.

First, this is all made possible because SteamDB keeps track of updates to all games, so as long as the version you want was available after the game has been publicly released, this should work.

Visuals: https://imgur.com/a/gBLJO

Go to SteamDB, and search up your game. https://steamdb.info/
Click on the app ID of the game you're looking for to go to its details page.
Take a look at the depots, and click on the depot ID of the one that looks like the one you want to download.
Click on the Manifests tab. Look at the list and find the version that you want to download. Record its manifest ID.
Open the Steam console. steam://nav/console
The syntax to the "download_depot" command is as follows: download_depot <appid> <depotid> [<target manifestid>] [<delta manifestid>] [<depot flags filter>] : download a single depot You only need to worry about the first three arguments to it. Type the command, then the app ID, depot ID, and the manifest ID of the depot version you want.
Wait for Steam to download the depot. You won't see any indication of progress, but you can tell it's downloading by looking at the network usage on your downloads page. The download can pause/resume if your connection goes out, but won't if you restart the client.
After the download is done, Steam will show you where the files were downloaded to.
Go to the game's installation directory, and move the files somewhere else. Then go to where the depot files were downloaded to, and move everything over to the game folder.
You may have to rename the game's EXE file if the dev changed the launch options recently. You can find the current EXE name by going to the game's SteamDB page and clicking on the Configuration tab.
You should now be able to launch the old version through Steam.
Note that game updates will make a mess of things, so if you want to stay on the same version, you should make a copy of the files so you don't have to download them again after Steam's done trying to update.