You may have made a mistake or applied an action that you come to later regret and wish to reverse the last action you had done. Maybe you simply want to add something and then check how it looks on your publication, using the undo and redo options can solve both these issues.
The easiest way is to simply click the left arrow or right arrow at the top right of the editor. The left arrows undo the last action and the right arrow redoes the last action.
Another method is simply mouse over the options drop-down menu in the top right of the editor. You will then see the 2 options named Undo and Redo. Simply click undo to reverse an action you did, or redo to re-apply an action you may have undone.