This article explains how to update usage.  To set usage based revenue, see this article: How to set up usage based revenue recognition; How to allocate revenue by percent completion.   

To update usage based revenue, go to Journals (top bar) and select Update usage based revenue.   

Link is here 

 From here, you view all line items from invoices in the account(s) selected as usage based. Filter at the top to choose:

• "All Projects" (default at top) or only view "Open Projects" (next button) to see only unallocated revenue.
• By account ("Select account" at top) or all usage based accounts at once (default view).
• Date range
• Enter data by monetary amount or by percent of total.

Then use the table to enter the amounts by month. 

Type the amount or percent in the white boxes under the months of allocated revenue. The final column shows the amount of unallocated revenue in the invoiced amount.