First, find that file (in my case, a .torrent file) and right click (or control + click) it. Select “Get Info” from the menu.

From this menu, click the triange beside the “Open With” pane to reveal the default application switcher.
Select your app from the drop down menu (I want Transmission) and then click “Change All“
Voila! Close the window!
Note: Oh, and if you only want that single file to open in that program, and all the rest to still stay in the other type, don’t click “Change All”. Just close the window.

What is the shortcut key to force quit an application on a Mac?

Short answer: ⌘ + option + escape

Long answer: Although many Mac zealots would say otherwise, Apple Mac computers are not perfect. Occasionally, applications will quit working and you’re stuck staring at what I like to call, “the Rainbow wheel Of Death”, or ROD for short. You’ll know him when you see him!

Often, a little patience will pay off and the application will resume, but if you know it’s crashed, or you’re impatient, force quit the application. Hit the key combo above, and the ensuing dialog will list your applications AND tell you if one is not responding. You can even force quit (or relaunch) the finder through the force quit dialog.

Alternatively, you can also find “Force Quit” under the apple in the title bar, or by holding the left mouse button on the guilty application’s icon in the dock.