If you are just wanting to credit a customer (sales) invoice click Sales, Credits & Refunds, New Credit Note, and select the invoice to be credited. On the next screen you will have an opportunity to edit the details if for example you only wish to make a partial credit or only credit certain items from the invoice. The credit note will have the same accounting transaction date as the original invoice.

If you just want to refund a customer payment (receipt), whether to correct an incorrect or duplicated payment posting or because you have sent a refund, click, Purchases, Payments, New Customer Refund, and enter the details on screen.

If you want to do both the above, for example to reverse a cash sale or credit a credit a customer (sales) invoice which the customer has since paid, click Sales, Credits & Refunds, New Refund, and select the invoice to be credited. On the next screen you will not only have an opportunity to edit the invoice details, but also specify the bank from which the payment was or will be made.

The above equally applies to supplier (purchase) invoices and payments but note that a New Supplier Refund is a receipt of money and will be found in Sales, Receipts.