You can cancel an iCloud storage upgrade but only within 15 days of buying:
Apple seem to have made this as difficult as possible. You may be able to do this using http://expresslane.apple.com - click 'See all products and services', then 'More Products and Services, then 'iCloud', 'Account and setup' then 'This topic is not listed'. Explain the situation in the box provided and wait for a response - include a phone number. However you may find you cannot proceed without entering a hardware serial number which is still eligible for AppleCare.
In that event you will need to phone them directly, using the Apple Online Store number given at the bottom of this page. The first person you speak to may tell you to do it from the iCloud website but this is not possible. Politely insist on escalating the issue to a more senior person and eventually you should be able to get through to someone who can deal with this (you may have to wait for them to call you back) - you will be asked security questions and for part of your credit card number to identify yourself.
If it is more than 15 but less than 45 days you may be able to get a partial refund by the same method.
If it is more than 45 days since you purchased the storage you cannot cancel it for a refund; you can downgrade your plan as from your next renewal date. Please see