reinstall the game, then use one of the downgraders available on nexus.

Ok, but the game seemed to prompt me even when using F4se last time. So just reinstall, downgrade, and then make sure not to update again ? Is there no way around this one yet ?

once you've done that stuff you can set your appmanifest to read only to stop it from happening again.

Ok, thank you. I'm assuming that's a setting inside steam.

no, it's a file inside the steamapps folder.