To correct a value in a cell, do one of the following
•To delete the old value and enter a new one, click the cell, type the correct value, and press Enter.
•To change a portion of the cell contents, double-click the cell, then use the arrow, Backspace, and Del to make the changes.
•If you have just typed a new value in a cell, and have not yet pressed Enter, press Esc to restore the previous value of the cell.
•If you have cut, pasted, or typed the data into a cell during the current work session, you can sequentially undo your actions.
Click on Undo located on the Quick Access Toolbar.