How to downgrade game bought on steam?

The only way to "downgrade" a Steam game is to download a Steam backup of the particular version that you want, delete your copy, install the backup and disable automatic updates.

To disable automatic updates for a game, right-click its name in the Steam client's Library tab, then do Properties -> Updates -> Do not automatically update this game.

Note that finding an untouched Steam backup of a game of a particular version can be quite difficult and sharing it may be illegal in your territory. If some of your friends also play that mod and haven't updated yet, you could ask them to make a backup and send it to you.