To cancel an order, go to the Orders list. Then search for the order you'd like to cancel and click 'View'.
Canceling an Entire Order
At the top of the order, click 'Cancel' and then click 'OK' to confirm the cancellation.
To cancel an order that has a cost associated with it but is marked as "Open" (i.e. check payments), click on the 'Refund' button at the top of the order. This won't actually send the funds back to the registrant (since there's no payment info tied to the order), but it will fully cancel the order.
To cancel an order that has a cost associated with it but is marked as "Paid" (i.e. credit card), and without issuing a refund, go to the ticket type near the bottom of the order, click on the dropdown arrow, then click 'Cancel'.
Canceling Line Items
To cancel a line item (not the full order), scroll down to the bottom of the order and next to the line item, click 'Cancel'.