This article covers how to edit a specific customer's usage based subscription from their profile. For directions on how to set up a new usage based subscription see this article.
Editing a customer's usage based subscription from their profile means the usage limit changes you make will be reflected just for that customer.
To do this, find a customer on a usage based subscription (click Customers > Customer List > search for a customer and select them.
From their profile, to the right of their subscription, click the dropdown menu and click Edit usage limits
Make changes and Submit.