When an Invoice has been generated for an Order, the originating Order Form will be locked from further edits.
Since Invoices and Orders are linked in Brandboom, it will not possible to edit the original Order Form independently.
To unlock the Order Form, you will need to remove (or refund if necessary) any payments on connected Invoices and delete the Invoices.
To avoid having to refund and re-charge your Buyer, you can simply remove a payment from an Invoice. After you've made the appropriate edits to the Order Form, you can manually log the original payment onto a later Invoice.