Download the attachment from your email to your desktop or any place you prefer. Once it's downloaded, Right click the file and choose open with. At the bottom of that box there should be an option to choose a default program to use. Select whichever program that will work and hit ok.
On a Mac, Option + Click on the file and go to "get info." There is a section in there that has "open with..." and you follow the same steps. At the bottom of the open with box there should be a button to apply to all files also. Be sure to hit that.