Each work item included in an invoice has its own Invoice Type; final or progress. By default, the invoice type is inherited from the work item’s status - "progress", if the work item is in progress, or "final", if the work item is completed. When the work item changes to the status “completed” it will automatically change to "final" invoice. All billable work for that job should now be completed, and the invoice is finalised and ready to bill to the client.
However, you are still 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.