Found under: Settings > Set Actions > Backup
To help you accidentally avoid losing data, you can backup you OpenSong/Sets/ directory.
When performing a backup, by default all sets are selected and they will be listed with any categories shown in brackets. You can select/unselect any set file as required using the check boxes.
By default the backup file will be saved to your OpenSong/Backups folder and will be named to include the date of the backup. In order to be recognised by OpenSongApp when restoring sets, the file extension should be .osbs. The file created will actually just be a zip file containing each set file, but the zip file uses this custom extension to make it recognisable by the app.
Click on the Backup button when you are ready. You will then be given options of sharing the backup file via email, or saving it to your Google Drive.
This option only exists from v6 of the app.