Icloud storage

If the purchase has completed, you can't cancel the purchase. You can downgrade your storage.

Downgrade your iCloud storage. https://support.apple.com/en-us/HT207594

Normally, all sales are considered final, but you can ask for a refund.

If you have received a confirmation email, click the Report a problem link at the bottom.

Purchases problem. https://support.apple.com/en-us/HT204084

If necessary, contact iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option.

iTunes Store - Contact Us - Official Apple Support. https://support.apple.com/en-euro/itunes-contact

iTunes Support – world wide https://support.apple.com/choose-country-region/itunes

Use the procedure I posted above to cancel the purchase and get a refund.

Request a refund for apps or content that you bought from Apple - Apple Support