When working on your design, you can always reverse mistakes at any time using the undo option. Find the undo option as an anticlockwise rotating arrow between add and the redo option on the top blue bar. You can also press Ctrl + Z(Cmd + Z on Mac) on your keyboard to undo an action.
To redo an action that you just undid, you can use the redo option between the undo and upgrade option. Find it as a clockwise rotating arrow. You can also press Shift + Ctrl + Z(Cmd + Shift + Z on Mac) on your keyboard to redo the action.