How to install an old version of a game in Steam

There are multiple guides for this already, but they’re just too insufferably verbose and long. If you need a long version, see one of them, like this one. Otherwise, here is a short version:

Concise step-by-step

1. Go to steam://nav/console (this opens the steam console)
2. Run download_depot [appid] [depotid] [manifestid]
   (more details, notably how to find those IDs, at the end of the post)
   NB: the download can be slow, and it won’t show any progress.
3. Note where the game was downloaded. It will typically be located in [WhereverYouInstalledSteam]\steamapps\content\app_[appid]\depot_[appid]
4. Find where the current game version is installed (in Steam client, right-click on the game → properties → local files → browse local files)
5. Replace the current game files with the newly downloaded ones (keep a backup if you want – although re-downloading the latest version is trivial)

Some more details on step 2.

The app ID and depot ID are technically useless since manifest IDs are unique, but Valve decided to make them mandatory anyway. I guess they just want to make sure running commands, while still possible, is as painful as possible.

For instance, to get the June 5, 2019 – 10:55:15 UTC version of “Love Thyself”, you’ll run
download_depot 949060 949061 7335981433319872232
and it will be downloaded into
[WhereverYouInstalledSteam]\steamapps\content\app_949060\depot_949061

The app and depot IDs can be found here: https://steamdb.info/app/949060/depots/
And the manifest IDs can be found here: https://steamdb.info/depot/949061/manifests/

Other example, Postal Redux from 29 May 2018:
download_depot 401680 401681 4007769963011890934
List of depots: https://steamdb.info/app/401680/depots/
List of manifests: https://steamdb.info/depot/401681/manifests/

Update (2020-12-22)

It seems that after a while (and/or after a certain number of newer versions have been published), old manifests are removed. For instance, I tried to download a manifest from a bit more than 2 years ago, which had 5 newer manifests: it wasn’t available anymore.

Update / bonus (2022-05-20): how to install a demo if you own the full game

Open the Steam console (cf the beginning of this post) and type app_install [app ID of the demo]. NB this is not the app ID of the full game, it’s a different one, which is usually rather close and which you can find simply by inspecting the “download demo” link on the game page, or by looking up the demo on SteamDB.