To cancel your pre-order, you'll want to open up iTunes on your computer, then follow these directions:
Choose Store > View My Appie ID, then click Manage Pre-orders, which located in the Settings section at the bottom of the Account Information screen in iTunes on your computer.
From the Mange Pre-orders screen, click the Cancel button to the right of the pre-order item you would like to cancel. You'll be asked to confirm your pre-order cancellation.