While the answers are (probably, I didn't check) correct, the sane way to modify file associations is with the GUI Microsoft designed for it. (That is, if you are a user and not a setup developer.)
Here is one tutorial with nice pictures, I found it with google: http://www.7tutorials.com/how-associate-file-type-or-protocol-programhttp://www.digitalcitizen.life/how-associate-file-type-or-protocol-program
But you can just open start (win7 menu or win8 screen, doesn't matter) and type file association, enter and it should open the correct control panel link. (Control Panel\Programs\Default Programs\Set Associations).