Note: Backup files are in the format of mapname_dd.mm.yyyy_hrs.mins.secs.ark
1) Stop your server.
2) Open your File Manager and browse to the following directory \ShooterGame\Saved\SavedArks\
3) Rename your main save file to something else, eg TheIsland.ark to TheIsland1.ark
4) Rename one of your backup files to TheIsland.ark
5) Start your server.