i want to cancel my preorder

You would need to contact support. 1-800-4my-xbox if you're in the US/Canada or http://support.xbox.com/contact-us

There is no option to cancel preorder.

This could be done on Xbox Support: http://support.xbox.com/contact-us

If it doesn't give you the option, you'll want to contact support and they should be able to cancel it for you. http://support.xbox.com/contact-us

1. Sign in to your
   Order history with your Microsoft account. If you have more than one Microsoft account, sign in to each one to check your order history.
2. Do one of the following:
   • If you want to cancel the entire order:
     
     Find the order or pre-order you want to cancel, and select Cancel item next to the order.
   • If you want to cancel an item in an order:
     
     Find the order or pre-order you want to cancel, select Cancel item next to the item, select the check box next to the item, and select the
     Cancel item button.
3. Check your email inbox for an email confirming the order was canceled. If you don't see the cancellation confirmation in your inbox after a few minutes, check your spam or junk mail folder. If you still don't see a cancellation confirmation email after
   waiting awhile,
   contact Microsoft Store support.