it wont let me downgrade my icloud storage plan

See instructions in Downgrade or cancel your iCloud+ plan - Apple Support including the following:

Tap Done. If you can't tap Done, make sure that you're signed in with the same Apple ID that you use for your iCloud+ plan. You can also try following these steps on a different device. If you still need help, contact Apple Support.

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