To edit the a purchase order, click the Edit icon (pencil & paper), under Actions in the row of the PO you would like to edit.
You will be directed to the detail page for that purchase order. On this page, you can change the Order Quantity or Unit Cost of an item by double clicking the field you would like to edit.
You can also add new Purchase Order Comments at the bottom of the page. If you are finished editing your PO, click the blue Save Purchase Order button at the bottom of the page.