The old way of downgrading using steamdb was removed, but it's still doable using a third-party program.

First you have to install Microsoft's .NET Desktop Runtime from this link: https://dotnet.microsoft.com/download/dotnet-core/3.1

Next, download DepotDownloader: https://github.com/SteamRE/DepotDownloader/releases

Open the command line (PowerShell for Windows 10) in that folder. In Windows 10 you shift + right click in the folder to bring up that option. To get the version of Fishy2 two before the current one, which is the fastest version, type in the following command. It should go without saying that since this is a paid game, this won't work unless you've already bought the game on your Steam account.

dotnet DepotDownloader.dll -app 1216540 -depot 1216541 -manifest 6130279695688438643 -username INSERT YOUR STEAM USERNAME -password INSERT YOUR STEAM PASSWORD

Wait for the download to complete - it might take a while. It's worth noting that most of the files are contained in one very large file, so the download may appear stalled at a low percent completion for up to 25 minutes depending on your internet speed.

Once you have the files, follow step 9 and onwards of this guide for the old method. https://www.reddit.com/r/Steam/comments/611h5e/guide_how_to_download_older_versions_of_a_game_on/