How to refund a fully paid customer invoice?

In Odoo, it's not possible to modify an invoice that has been validated and sent to the customer. If a mistake was made on a validated invoice, the legal way to handle that is to refund the invoice, reconcile it with the original invoice to close them and create a new invoice.

Refunding fully paid invoice
If you need to refund an existing invoice partially, use the Refund Invoice button on the invoice. In the refund method field, select "Create a draft refund".