Click on the link to read how to "Downgrade or cancel your iCloud storage plan" on an iOS device, a Mac, or a PC. --> https://support.apple.com/HT207594
I assume this was set up under a different account from that you are using now? You don't need access to the actual email account but you will likely need to be able to remember the email address which you use as your Apple ID for the account.
If you need help, contact Apple:
Click here --> https://support.apple.com/choose-country-region Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service --> "See a list of Apple phone numbers around the world."
Click here --> https://support.apple.com/HT201232