Your original photo is never overwritten. Every edit is saved as a new version alongside it, so the photo you uploaded is always still there.
To restore the original, open the photo and pick the original version. Nothing needs undoing. Nothing needs to be undone and nothing is lost by trying an edit you end up not liking.
To redo an edit you did not like, run it a second time. Results vary between runs, so a second pass on the same photo often lands differently. Each run costs its own credits, because each one is a new generation.
To try a different look instead, run a different edit on the original rather than on the edited version. Edits stack if you apply them one after another, which is occasionally what you want and usually not.