Are you on Windows or Mac OS?

For windows, select one such file on your desktop & go to File>Properties & press the Open with ... button & then select your app. In this case that would be QT.

For Mac OSX, highlight/select such a file & with either CMD+I or Control+click for the menu to come up & select Open with ... & select your app. OSX will make an educated guess as to what apps should open it but if your prefered choice is not there (& that can happen) then scroll all the way to the bottom of the list & choose Other ...

In Mac OS9, you'll need to go to your Apple Menu>Control Panels>File Exchange & scroll down the list until you find that type of file. If it's not there you can add it. Then select it in the list & open the advanced section & choose your prefered app to open it. That will mean directing the chooser to your applications folder to choose it. (It must be 18 mths since I last opened it lol)

Now whichever app you've chosen will become the default app to open that type of file.