To cancel and/or refund an order, log in to the mypreorder Merchant Portal back office:

Go to Reports.

Use any of the Orders reports to locate the order by date, customer, email etc.

Use the Actions dropdown next to the order to cancel the order and/or perform a refund:

Note that refunds are sent back to the source of the funds, i.e. if the customer paid by their credit card, the refunded amount will be refunded directly to their credit card.