You would have to delete your songs one at a time. Note that if you do this, any set lists you've created will be emptied out, so you will probably want to delete those, too.
If your goal is to start over with all new songs and set lists, it would be easier to create a new database and import into that. However, you will have to set again any settings that you were using in your old database.
If your goal is to update your songs, you will have to do that in the app, because importing the same song again will not update the previously imported song.
If your goal is to add songs to your database, you can simply import again and only the new songs will be added.