There are a few ways to cancel an order:
1) From the Dashboard:
Navigate to Dashboard, and scroll down to the Open Orders section where you can find all open orders. Click the "X" icon next to the order you want to cancel.
2) From the Trade tab:
Similarly, open orders can be found within the Trade tab.
3) From the chart:
As a convenience, you can also cancel an order directly from the chart, by clicking the "X" button next to the order shown for the selected account, and pair.
In all cases, the request is immediately sent to the exchange where your account is held and a confirmation is displayed upon successful order cancellation.