HOW TO - Make changes to a Sales Order which has been shipped
From Adjutant Wiki
How to change the qty or price on an order that has been invoiced
1. Open the invoice associated with the sales order
2. Void the invoice by clicking the 'Issue Credit for Invoice' button on the left side of the screen. Click through the prompts, selecting Yes. This will unbill and unship the sales order.
3. Open the original sales order and make specified changes to quantity, items, or pricing.
4. Once changes have been made, re-ship/invoice the sales order.
NOTE: This action is only available for invoices which have not been paid