you can use the "open with" context menu and pick a new app to open it with. You have the option of selecting that app to be the "designated" app too by checking the "use this program to open next time" box.
Otherwise, just look in your reg for the extension and remap it as necessary, but if you're a n00b to regedit, just stick with #1, it's much easier.
-=bmacd=-
edit: 10 seconds too slow