First open invoice view that can be found on Main, Case, Client or Payer Card, you should now see one or more draft invoices (if you don’t have any draft invoices, please follow this article on How to generate draft invoices)
Under “Approve” tab you will see list of approve invoices, select “Unlock” button
Now Approve invoice will fall back under “Drafts” tab.
Note: drafts invoice in draft state allows user to edit it.
(Hear are link of how to edit draft invoice)