Did you save an
aia source code file to your desktop? Did you save as an earlier copy on the MIT server t using
Projects > Breakpoint or
Projects > Save project as.. ?
If the answer is no to all of these, it is impossible to revert to a previous version. App Inventor has no unDo function.
If you have an aia, go to Projects > Import project (aia) from my computer , select your aia and you can restore to an earlier version.
If you do not save using one of the above methods and then make a mistake and corrupt your code . you have to figure out how to undo the problem coding to revert to a previous condition . Of course, you could also start all over. The second time should be easier because you have learned to code what was necessary.
Regards,
Steve