I want to cancel my iCloud storage plan. How do I do that? Thanks

Are you certain it isn't already downgraded but not yet taken effect? See the following from Downgrade or cancel your iCloud+ plan - Apple Support

If you downgrade or cancel your iCloud+ plan, the change takes effect after your current subscription billing period ends.1

If that doesn't help, see the following from that same support article:

If you still need help, contact Apple Support.

Cancel upgraded iCloud storage by downgrading to the free 5GB plan. See instructions in: https://support.apple.com/en-us/HT207594