Read this support article to learn how to...
"Downgrade or cancel your iCloud+ plan" on an iOS device, a Mac, or a PC.
Click here --> Downgrade or cancel your iCloud+ plan - Apple Support
"If you can't click Done, make sure you are signed in with the same Apple ID as your iCloud storage plan. You can also try following these steps on a different device. If you still need help, contact Apple Support ( https://getsupport.apple.com/?caller=cups&PGF=PGF63006&category_id=SC0060 )."
For some people "Done" does not show, in which case a person found this solution worked for them: iCloud Downgrade Lösung - Apple Community = 'Click on the same option you currently have, then click accept. Then go back to the selection window and select the downgrade you wish and click accept.'
if that does not work, contact Apple:
Click here --> Choose your country or region - Official Apple Support Select your country, then a product.