Want a smaller iCloud storage plan

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

Click Done. If you can't click Done, make sure that you're signed in with the same Apple Account that you use for your iCloud+ plan.

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

This is primarily a user-to-user support forum; other Apple users like you.

You can also contact Apple Support as described in Contact - Official Apple Support

How recently did you upgrade to the 2TB plan (as described in your original post)?

If you go (on your Mac) to Apple menu > System Settings > iCloud and look just above the bar graph at the top of that screen where it shows "Storage: x of y GB" -- what are the numbers there?