Once orders have been created in Airsquare, there's no 'edit' function to add/switch items.
Invoices, payments and 3rd party systems are directly tied to an order.
If you need to adjust an order, you need to return and refund the item(s) and then create a new order for the new items. There is an option to create a credit note for the refund in Xero.
You can use the store credit option to allocate credit for the person/organisation which can then be used as payment on the new order you create.