I am not able to download the Studio Pro, so I want to keep using the Studio. How do I put the Mendix version back to 9.21? (I am working alone on a app so it does not matter if it does not synchronise etc.
Create a branch from the revision that you want to use for Mendix Studio (in your case, the app before upgrading from 9.21 to a higher version)
Download the branch.
Go to your File Explorer and delete all the contents of the mainline folder, but keep the .svn folder (Might be hidden based on your File Explorer settings)
Copy all the contents of your branch folder into the mainline folder, except the .svn folder.
Commit your mainline folder using TortoiseSVN or a similar tool. Do not use Mendix Studio Pro to commit, this will not work.
After this, you will have your 9.21 branch available in Mendix Studio.
If it doesn’t work, open your mainline in Studio Pro and make a small change so you can trigger a commit in Studio Pro. After this commit, Mendix Studio should work.
Hope this helps!
While not the answer to the question you are asking, it’s worth pointing out that Mendix now have a portable version of Studio Pro that doesn't need admin rights to install. This is only for the very latest versions of Studio Pro though. This should allow you to run Studio Pro even if you don’t have admin rights on your computer.