These instructions apply to templates that use the Classic editor, as well as emails and newsletters.  

You can use the keyboard shortcut CONTROL + Z (Command + Z on a Mac) to undo any changes you make to regular text in the editor.  To redo a previously undone change, press CONTROL + Y (or Shift + Command + Z on a Mac).  Undo and redo are browser features, so they will not be able to be applied to some actions.

Note: The undo and redo features are available for items created in the Drag and Drop editor.