If showing a group use the Group Controls undo
Ctrl-Z if a diagram has the focus
If you add or paste objects, the Undo button removes them
If you add, delete or change connections undo changes them back
If you have deleted objects whilst a diagram is open then made other changes, you can still recover the deleted objects with the List View
The matrix form has an undo button and also Undo All Changes. You can undo most major actions - delete rows or columns, add columns and rows.
Producing a decent undo function has been a big problem whilst developing ControlDraw. It would be good if undo worked like it does in Word etc, with multiple levels.
However, this is a major task, essentially because Controldraw is a database and one action generally affects several tables. For example Access does not do it at all well either.
Controldraw generally provides a single level of undo and warns if an action cannot be undone.
Remember - ControlDraw is a database, not a drawing package.