Session Based: Your undo history is active for your current session. If you refresh the page or close your browser, the history will reset.
Plan Based: You can only undo/redo actions from within a single plan.
Successive Steps: You can undo multiple steps in a row, allowing you to roll back an entire series of changes if a design isn't heading in the right direction.
Click on the relevant icon to either undo or redo an action.
Or use keyboard shortcuts:
Undo: ctrl+z for Windows or cmd+z for Mac
Redo: ctrl+y for Windows or cmd+y for Mac