You can not downgrade from the latest commit, but you can look for the latest commit that was made in the previous version, and make that your new main-branch:
This assumes that there is no inrevertible change made to your database.
If any irrevertible change is made to your database, you also need to restore the backup that was the last one to be in 9.0.5
Downgrade the app version is not possible in mendix studioPro version.
You need to download the 9.1.1 version and export all the modules from application. In 9.0.5, create the new application & import the modules from 9.1.1 and work on it. That’s the solution.
From the source control, you can reverse to the version you created.