It happens to the best of us. You may delete the wrong value, replace the wrong formula, or enter something incorrectly. In short, you make a mistake. Excel allows you to undo almost any of your editing actions. The easiest way to undo an edit or formatting change is to press Ctrl+Z. Excel very helpfully undoes whatever it was you just did.
Excel also provides an Undo tool at the upper-left of the Excel program window, just above the leftmost tabs of the ribbon. This tool looks like a curved arrow pointing backward-to the left. If you click on the tool, it is the same as pressing Ctrl+Z. If you click on the tool and you hear a “ding,” it means there are no actions to undo. If there are more than one actions that can be undone, click on the down-arrow to the right of the Undo tool and you can see the various actions that you can undo.