is there an undo in the editor?

I find myself not able to use the editor very well. How do I undo a screwup?

There's an undo button (showing a sideways u-turn arror) that undoes the last changes. Thsi is usually sufficient to get rid of screwups.
If all fails, you'll need to reload the song from the forum archives.
There is also a 'retore all exercise songs' under the 'more' menu.