Edit Order After Invoicing

To edit an invoice AND affect inventory you must first delete the invoice (which reopens the order) by going to Income -> Invoices, select the invoice you'd like to modify.  At the top of the page, click Edit, and then click 'Unpost'.  With the invoice now unposted, click the Delete button.

Go to Income -> Orders and search for the order number or customer's name.  Re-invoice the order after making changes.