From the Testing forum:
Downgrade process (server)
Stop the UrBackup server, restore
C:\Program Files\UrBackupServer\urbackup
or/var/urbackup
from a backup before upgrade and then install the previous version over the beta release.
Be sure the database backup you restore is from before you upgraded to the 2.4.x version. If you’re not keeping separate copies of the “urbackup” folder/directory from your Backup storage path (Settings: Server) then you get to reinstall with default settings and start over with the experience you gained the first time.