Yes, you can change your booking using 'Manage Booking', which is available on the app or mobile site version of qatarairways.com.
Click the link below to Manage Booking (you will require PNR/ticket number/Privilege Club number and passenger's last name)
https://booking.qatarairways.com/nsp/views/retrievePnr.xhtml
For assistance contact your local offices, contact details can be found in the following link https://www.qatarairways.com/en/worldwide-offices.html