Undo an edit by tapping 
and selecting Undo. Select Undo multiple times to back up multiple steps in the workflow.
Redo an edit that has been undone by tapping and selecting Redo. If multiple steps have been undone, then selecting Redo multiple times will bring back those steps in the workflow.
Revert back to the unedited image and start again by tapping and selecting Revert. The Revert action can be undone by tapping and selecting Undo.