Once an order is sent to you, you have the ability to edit the quantity of the items ordered.
Find the order you would like to edit and click anywhere within the Order to be taken to the order details page.
Under the Items Tab, click on Edit Quantity and adjust the quantity of the items ordered and click Save
Please note that this action is ONLY adjusting the quantity of items ordered. The order will remain in Pending status until the Accept or Reject button is clicked.