That all depends... exactly what are you trying to do?
Go from SVN back to a release version?
Check your server's gmod/data/ULib folders... should be users_backup.txt and groups_backup.txt
No guarantee depending on how much you've edited manually since going to SVN, but those files, at least for me, look to be backups of the release version files.
If that's not what you're trying to do... what help do you need?
