You have to uninstall 12 first.
I would strongly advise against trying to do it manually. If you want to go that way, first completely shut down FMS:
sudo launchctl stop com.filemaker.fms
Wait and confirm with Activity Monitor that there are no more processes running that start with "fms"
Then copy your files and backups to another location
Then use something like AppCleaner to delete the FMS folder
You may have to do some manual cleanup as well.