There is no Revert. While you are still in the Editor, a series of undos takes you back through your changes. Once you save the song, its previous version is gone, and you would have to import the original again from a backup file or from the Forums.

 

When you want to try out an idea, use Duplicate first and work on the copy, so the original cannot be overwritten.

 

For more, see Create a song on the iReal Pro website.