To cancel your booking and get a refund of your ticket price (depending on the fare conditions of your ticket), please contact the agent/office where you booked your ticket, or if you changed your booking, contact the office where you last modified it.
For bookings made (or changed) at qatarairways.com, you can request a cancellation or refund at http://www.qatarairways.com/global/en/refund-request.page.