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 and selecting the invoice you'd like to modify.  At the top of the page, click Edit, and then click Unpost.  After the invoice has been 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.