make sure you are working on the actual executable file (not a shortcut to it) then you should just be able to delete the existing extension and type in the new one. you may have to change the options you have set for displaying known file extensions,
i dont really understand why you would want to do it though.
If you change a extensions it will not work, but you can change what it opens with. By go to Explorer click the file then hold the shift key down and right click the file, choose always open with and put a tick in the box
This thread is now locked and can not be replied to.