You can use this API to update the value that a counter has for an allowance. You'll need to enter the subscription ID and and the counter ID, which you'll find on the allowances page in the Billsby app. You'll also need to let us know the operation type, so whether you want to increase, decrease or set the allowance at a specific number, and the value by which you want to change (or set) it.
You'll want to use this when your customer uses up their allowance for their subscription. You may want to first check if your customers have any allowance left on their subscription - using the Get counter API - and then use this API to update their allowance if it was used.
As a note, counter updates are queued, so you may not see changes reflected instantly if you call to get the counter data straight away.