Copying the files

You've successfully downloaded the older version, now it's time to actually make the changes.

When the download finished, you would have seen a message show up reading something like the following.

Depot download complete : "C:\Program Files (x86)\Steam\steamapps\content\app_620980\depot_620981" (327 files, manifest 1175216641760077721)

To finish the process, just copy everything from the path in the message on top of your Beat Saber install, overwriting all conflicting files.

Even though there's a decent chance of things working, I'd strongly recommend deleting your Plugins folder and reinstalling all your mods with ModAssistant.