two ways:
open a folder and click on the tools>> folder options
click on the file types tab and select the file type you want to edit.
in the bottom where it says details and "opens with" click on the change button.
find the program you want as the default to open a file with that extension.
-----------------------
another way is to hold down the shift key and right click on the file and select "opens with"
make sure the "always open" checkbox is marked. and select the program to open.
