Why can't I downgrade from the Billing section?
To downgrade your subscription, please make sure to delete the users first.
You can delete users by navigating back to to Users section.
For example, if you have 25 users configured, but would like to decrease to 20 users, you must delete five users from "Network Name>Users".
Now you can go to your "Billing>Modify subscription" and decrease the number of users to 20.