Get Info for a file you want to change the association on. Open up the Open With section. Pull down the list. If the app you want is listed there, select it. If not, click Choose Application and find the app (If the application isn't registered as being able to handle this type of file, but you know it can open it, you'll have to select All Applications instead of Recommended Applications to get everything un-grayed). If you want this change to apply to all files of this type click Change All and confirm that you want to do this.