If you're asking how to apply a credit note to an invoice, you can do it by following the steps outlined below.
First, we'll want to make sure you create a credit note so yo have something to match with your invoice. In case you haven't done it yet, here's how:
Go to the + New icon.
Select Credit note or Give credit.
Enter the customer’s name, then fill out the required fields: Date, Products/service items, Quantity and Amount.
Select Save and close.
Then, you can apply it to the invoice. Here's how to manually apply a credit note to invoices:
Go to the + New icon.
Select Receive payment or Receive invoice payment.
Enter the customer’s name, then fill out the required fields. Leave the fields Paymentmethod, Reference no, Deposit to, and Amount received blank.
In the Credits section, select the credit note. Then, in the Payments column, enter how much credit your customer want to use. Make sure the total to pay (if any) is correct after applying the credit note.