You can cancel or change the type of issued ticket from the order page:
Clicking "Edit ticket" opens a form:
If you change the type, the corresponding sales and availability figures of event ticket types are adjusted accordingly. To send the updated ticket to the buyer, use "Resend unused tickets" on the same page.
When cancelling, the event availabilty is increased by one. If your event uses reserved seating, the seat will be released as well.