Any operation that modifies either the song or the patch can be undone via Edit/Undo or Ctrl+Z, and redone via Edit/Redo or Ctrl+Y. Undo is unlimited. Note that the song and the patch have independent undo histories, and consequently the undo/redo command's action depends on which window has focus, as shown below. To undo/redo a song edit, first make sure the chart view has focus, otherwise you may inadvertently undo/redo a patch edit.

Chart viewUndo/Redo the most recent song edit.
Any other windowUndo/Redo the most recent patch edit.