It will not let me downgrade my storage

Cancel upgraded iCloud storage by downgrading to the free 5GB plan. See instructions in the following Apple article: Downgrade or cancel your iCloud+ plan - Apple Support

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

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