When an invoice or an invoice line get the status definite, it is not possible anymore to modify the invoice. However, there is a possibility to correct a definite invoice. This happens in the form of a credit invoice. Just as a normal invoice, a credit invoice consists invoice lines.
Click 'Financial' | 'Invoice' and go to the tab 'Definite'
Open the desired Invoice
Click 'Credit' and click [OK]
A credit invoice is now made with the next invoice number.
The original invoice can you see at the general tab of the credit invoice.
Go to the tab 'Concept' and set the credit invoice definite.
Credit invoices can't be deleted, even if they have the status concept.