How to downgrade FCPX from 10.2 to previous version?

Final Cut Pro X 10.2 keeps crashing after trying to convert the old libraries. It starts if I use a fresh library, but I can not access the old ones, which I'm working on.

Removing Fonts as a fix does not help.

How do I get back to the old FC Version 10.1.8.?

I do have a backup of the entire maschine on a backup drive.

Which files do I have to copy?