By default invoices are generated as "Final" ready to be sent to clients.  However, if a discrepancy is found during review,  notes (time/expense entries) in the invoices can be edited and those edits are automatically reflected in invoices.  The invoices simply need to be downloaded again after making edits in notes.

New Invoices in Draft mode setting under Invoice/General Settings can be used to generate invoices in Draft mode to easily identify which invoices have been reviewed.  The invoice PDF will show "DRAFT INVOICE" caption on its face. To change an invoice from draft mode to final (or vice versa), under Unpaid Invoices section on the Dashboard, click on "Is Final" checkbox for a particular invoice.  Same checkbox is also available under Clients -> Invoices and Cases (Projects) -> Invoices tab.