Dessin stores history of actions executed by the user and allows to quickly revert any action.
In order to do so, touch any free space and swipe to the left. You will hear undo sound and the action will get reverted.
You can also re-do any changes that you reverted by touching free space on the canvas and swiping to the right.
If your device supports multitouch you can use several swipe menus simultaneously to undo multiple actions at once.