See the following if you want to delete iCloud Backups that you no longer need: Manage your iCloud storage - Apple Support
Cancel upgraded iCloud storage by downgrading to the free 5GB plan. See instructions in: Downgrade or cancel your iCloud+ plan - Apple Support including the following:
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