dyung welcome to the QR forum. You may find the dashboard a useful tool for exploring the forum.
QR are not known for their willingness to change flights.
You need to look at the Fare rules that are now supplied as an attachment to the "Booking confirmation : xxxxxx" e-mail. They can say "change fees are nnnn" or "changes are free of charge". This very much depends upon your booking class and country of origin of the first flight on your booking.
Timing is everything when changing flights. If you change before the first flight then you will also have to pay any fare difference that may occur. After you have taken the first flight then you will not be repriced on the changes flight (DOH-BKK). But there may be a fare difference for a change of ticket class if your original is sold out on the new flight.
So you need to do it in the lounge in DOH.
Any change within three hours of departure will be regarded as a cancellation and new purchase. Thus you should try as soon as you can after arriving on your incoming flight.
This reply is very general because you supply no details but it should cover your particular problem.