When an invoice is FINAL, it cannot be changed anymore. Any corrections needed will have to be done through a Credit Note.
This is a new invoice which is linked to the original invoice and can be seen linked under the main invoice.
Invoices can be fully corrected by creating a full Credit Note.
Via Desk:
Go to the invoice you’d like to credit
In the menu, select ‘Credit entire invoice’

Add an optional note and create the credit note

The credit note page will be created, the user is navigated to the credit invoice where the PDF can be downloaded. The original invoice is linked to the credit invoice as shown below

Once a credit note is created on the invoice, the main invoice remains in FINAL and the new credit invoice, denoted by a ‘CR’ in the prefix, is created in DRAFT status
The Credit invoice is also linked in the main invoice

Via the API:
Invoices can also be partially credited, by specifying the invoice lines and the credit amount in the credit action on the invoice.
Via the API:
To read more about creating and managing credit notes via the API, read our documentation here