Found under: Settings > Set Actions > Restore
If you have created a backup of your sets, you can load these back in to the app at any time (only available since v6).
When you select this option, you will first be presented with a file chooser to select the OpenSongApp backup file (this will have the .osbs file extension). Once you have selected the backup file, the app will show you a list of all of the set files that it contains.
By default all sets are selected for restore, but if you want to overwrite any existing set files you have with the same name, you will need to switch on the Overwrite existing item option, otherwise existing sets will be ignored.
Click on the Import button to copy the set files from the backup into your OpenSong/Sets folder.