How to reject customer invoice, which is not issued to customer?

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.

If you need to cancel an existing invoice, use the Refund Invoice button on the invoice. In the refund method field, select "Cancel: create a refund and reconcile".