I rang up the wrong order and the ticket printed. How do I undo it?

If it was not paid for, a manager voids it with a reason and the void is recorded. If it was already paid for, it is a refund rather than a void, because those are different events with different money and letting one quietly undo the other would leave the drawer wrong. Reports, Shifts and the POS counts sales cancelled after printing, which is a number worth glancing at now and then.