Each work item included in an invoice has its own Invoice Type; final or progress. By default, it is inherited from the work item’s status, when the work item has the status “Completed” it will automatically select Final invoice. However, you are able to change the invoice type to progress type if needed.
To do so, open up Billing from the Main Menu and choose Invoices. Pick the invoice you'd like to edit and on the right-hand side you'll see the option to change the invoice type.