How to modify the confirmed 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.

Modifying a validated invoice:
If you need to modify an existing invoice, use the Refund Invoice button on the invoice. In the refund method field, select "Modify: create a refund, reconcile, and create a new draft invoice".