This is already covered in the GenP Guides in two places: once in the main GenP Guide steps themselves, and again in the Troubleshoot Section.

If you had referred to those, you would have found two links that direct you to Adobe’s own page explaining how to roll back to, and install, previous versions of apps here.

Additionally, this topic has been addressed in the Announcements channel in GenP Stoat. As noted there, you also have the alternative option of replacing Photoshop with a pre-release GenP test version instead of rolling back to the previous release. This is also mentioned on the Compatibility List.