1. Uninstall the current version of the game.
2. Delete the folder "The Talos Principle".
3. In the games list, right click on "The Talos Principle".
Select Properties, then the Betas card.
4. Now you can choose the version you want to install.

(Don't forget to unsubscribe all maps of the current version.
Otherwise you will get an error message.)

Unfortunately, that's the only way it works. I had the same problem too.