Instead of cancelling an open order and placing a new one, if you want to change just the quantity or price, you might be able to modify the order.
To modify an order, go to the Orders Page, select Open or All Statuses from the status dropdown, and click on the Modify button for the order you want to modify.
A pop-up will appear, showing information about the order.
On this screen, you can change the quantity and the price(s). When done, press the Modify Order button.
A notification will appear that the modified order was submitted.
When the order is successfully modified, another notification will appear informing you of the success.
The actual information in the Orders Page table should always be used as the final reference. Until the Orders Page information has been changed, do not assume the order has actually been modified.