Changing the default application!
For each file type, OS X has a default application that it uses when you double-click on one of these files. I probably mentioned one or two ways of changing that default application, but I think that this is one of the easiest and most efficient way. You simply have to select a file that has the extension that you want to make the change for and press "Command - I" or click "Get Info" in the Finder. The interesting section of the Get Info window is the Open With. From there, you select the new application that you want by default. It will only change it for that specific file, but you can apply it to all by clicking on "Change All...".