[Observium] Billing module - editing a bill (specifically "Allowed") property

Evening all and Happy Halloween :-)

Reading the doc here "http://www.observium.org/wiki/Billing" and searching through the site I could not find this or reference to it.

I have a customer that had 10Mbps set, he had a large overage last month and wanted to see this month if his usage will go up ( as we charged overage last month ) so next month he wanted to change the 10 to 20, so he can "suck it and see".

I went into the "Edit Bill" and I see plenty of options : Billing Type, Description, Billing Day etc. But I cant see where I can edit the 10 and change it to 20 ?

Thoughts?

I am running : Observium 0.13.10.4670

Thanks, Chris