On a reservation order you can cancel one or more tickets or the entire reservation. This can be done through the view order page (see more here).
Cancel one or more tickets of a reservation
If you want to cancel a single ticket (or tickets) from a reservation order, you can select it from the "tickets" table and click on the "cancel" option below the table.
Cancel the entire reservation order
Click on the "cancel order of sale" option at the right bottom of the order page.
Do not edit reservation orders made online before contacting the customer.