Navigate to the POS section. At top of the page, click Open Sales. In the popup window, search for the sales, that you would like to edit and click View/Edit Order button.
Edit Open Sales
You can add new items onto the same order in this new page. Note that the initially sold items in the same order cannot be removed or edited. Once done, click the Update button to post this edited order. The captain order printed after editing this order will contain only the added items.
Update open sale