Does anyone know on fsx how to change default air traffic to desired ones like Qantas and Singapore Airlines?

Yes. You can spend hours browsing the various flight sim sites such as simviation or avsim for texture repaints of the default aircraft. Usually the downloads give instructions on how to apply the new textures, and it often means making an edit to the aircraft cfg file, but it's not difficult, usually just a quick cut and paste job. Any of the FSX forums should provide you help.



Have you also considered adding new airlines to the existing air traffic ? You can do that either by buying the FSX addon 'Traffic X' by just flight, or by visiting the projectAI site and downloading and installing their free air traffic packages for various airlines.



Links;

http://simviation.com/menu.html

http://www.avsim.com/

http://www.projectai.com/

http://www.justflight.com/developProduct