Applying Customer overpayments

[Alan Johnson]

Hello, I'm having an issue with GnuCash. A customer has a 
overpayment/credit balance. How do I apply it to an invoice so that it 
comes out of the AR report at the bottom of the customer report? The 
total due / sums are correct, but the customer report has a 91+ day 
balance of (0.03). I would like to apply the payment to an invoice / fix 
the AR box on the customer report. This is on version 2.6.19.

Thanks,

Alan