If you want to change your name then you would have to cancel existing PNR and book a reservation. As per , passengers are not able to make changes in name on the existing PNR. For changing name you can contact fr om nearest Qatar airways office wh ere you are able to change your surname supported by legal documents.
https://airlinespolicy.com/flight-change-policy/qatar-airways-flight-change-policy/
https://airlinespolicy.com/flight-change-policy/qatar-airways-flight-change-policy/