Issue a credit or a refund
You can choose to issue a full or partial refund, or issue a gallery credit that can be applied on a future invoice.
Should you need to issue a full or partial refund, you can do so either from an inventory line item or from the Payments section of the invoice. You'll have the option to select whether to issue a refund or a credit that can be applied on a future invoice.
For a quick overview of credits and refunds, check out this short video: