How do I change the fn key shortcuts?

Specifically I want to change fn+enter (the enter on the num pad). When I press fn+enter on my keyboard it opens the calculator, which I like, however I have just upgraded from windows 8 to 8.1.
In windows 8 fn+enter opened the original calculator that has been on windows for quite some time, but now with windows 8.1 it opens some new full screen calculator that makes doing other things at the same time frustrating.

I would like to change the settings so that fn+enter opens the old calculator, not this new one.
Both calculators are installed on the computer.
I tried changing shortcut key for the old calculator by opening its properties and entering fn+enter in the "shortcut key" box, but this just opened the new calculator.