Is this the complimentary storage because you had a MobileMe account and migrated it to iCloud? If so you need do nothing; it will downgrade to free 5GB automatically at the end of September. If you want to continue it you will have to cancel the downgrade which is already in place.
Or did you pay to extend the storage? You can cancel an iCloud storage upgrade within 15 days of buying:
Apple have made this as difficult as possible. However two posters have managed to achieve this. Elizellen was able to use Express Lane to get a refund:
However if you use Express Lane may find you cannot proceed without entering a hardware serial number which is still eligible for AppleCare. julia22 managed to find a way round that by phoning them - please see this post:
it gives the USA number - if you don't live there you will have to find an equivalent number from the 'Contact Us' link at bottom right of this page.
Poster 'CCrefund' has posted a method applicable to users in the UK: https://discussions.apple.com/message/21002309#21002309
If it is more than 15 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
and expand 'Downgrade your storage for the next plan year'.