Hello Arthataxac,

To apply a credit to an invoice, please follow these steps:

1. Select the + New icon
2. Select Receive Payment 
3. Select the customer's name
4. Tick the box next to the invoice you wish to apply the credit to
5. Tick the box next to the credit you wish to apply
6. If the credit is the only payment you wish to apply, ensure that 'Amount Received' is $0
7. Save your transaction

I hope this helps you apply your credit.

-Ken