This is user-to-user forum: you aren't talking to Apple here.
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'.