Q: How can I cancel a preorder?

There are instructions on this page for viewing and/or cancelling pre-orders : http://support.apple.com/kb/HT5714

You mean that it's now a purchase and not just a pre-order ? If you do then you can try the 'report a problem' page to contact iTunes Support and see if they will credit or refund you : http://reportaproblem.apple.com

If the 'report a problem' link doesn't work then you can try contacting iTunes support via this page : http://www.apple.com/support/itunes/contact/- click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption