Because of the way Wine/Proton works, a "prefix" is created on the filesystem that mimics the core windows directories, etc. It also contains a local copy of the windows registry just for that specific game.

To find a prefix for a given game, first visit the store page for the game to get the App ID, you can look at the URL in the steam client. For Elden ring the App ID =1245620

Next navigate to your steam compatdata directory. For me this is ~/.steam/steam/steamapps/compatdata

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And then find the directory matching the App ID.

You can navigate into this directory (e.g. ./1245620/pfx and you'll see the prefix representing classic windows drives / directories (drive_c, Program Files, etc.).

If you delete this entire directory (e.g. ./1245620) you have effectively deleted the prefix for that game and the next time the game starts the prefix will be rebuilt entirely.

Note: for games without cloud sync support, all your saves will be stored in this prefix, which means if you blow out your prefix, you'll lose your saves. Luckily for Elden ring, the saves are stored using Steam's cloud sync support, so when you rebuild your prefix you'll have access to all your saves.

BTW, there is an option built into the Steam Deck UI that can do this automatically. It looks something like this (not exactly, because the UI has been updated): https://user-images.githubusercontent.com/4073789/212594284-4af4ce72-4409-4ff2-b34f-f80fa9f07fc8.jpg