If in the US, follow the link below (you must request a refund within 15 days) -
it gives the USA phone 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.
Keeping current plan, but downgrading when it ends:
Settings>icloud>Storage & Backup>Change Storage Plan. Look at downgrade options. There you can choose the plan you want when the current one ends.
Anyone in the UK who stmbled into this thread,
looking for the answer on how to get a refund on an unnecessary iCloud storage
upgrade, clik on this link:
You only have 15 days to do this after purchase.
I found getting to the right place a puzzle that might stump a genius
at cryptic crossword puzzles.