If your ticket status is Ticket(s) Issued, you cannot cancel your order, please see the question Can I get a refund if I cancel my ticket?.
If you haven't pay for the ticket, you can cancel your flight reservation by clicking on the Account tab located on the right-hand side of the navigation bar at the top of the page. Sign in and select My bookings, then Flights to see your flight order history. Click on the relevant order number to open the booking details page. Click the Cancel order button to cancel your reservation.
If you have already pay for the ticket, the ticket is being issued and cannot be canceled directly. If there is Cancel order button on the booking details page, you can apply for a cancellation, but whether the order can be canceled successfully or not is subject to the page prompt and the email notification. If you haven't received the email to inform you that the cancellation is successful in 30 minutes, please check your order status again to see whether it has been changed to Ticket(s) Issued.