The Undo and Redo commands are located on the Home tab.
To undo the previous action to your visual, click Undo from the Home tab, or hold down the following keys from your keyboard simultaneously: Ctrl and Z. If you would like to redo an action, click Redo, or hold down the following keys: Ctrl and Y.
If the Undo or Redo buttons are gray, meaning unable to be selected, there is no action to undo or redo on your current document.