Yes, you can cancel your orders. When you submitted an order and you decided that you want to make changes or do not want to proceed, you can always cancel your order. To do so, you need to go to your MARKET page, then scroll down at the bottom of the page and look for My Open Orders box. There is an option to cancel a specific order. Just click "CANCEL."
However, once the order has been completely matched, it will disappear from My Open Orders box and will be recorded in your Transaction History. It means that the transaction is completed. Therefore, you CANNOT cancel your order anymore.
*** If you successfully canceled your order, your funds will be returned to your account.