Hi flynby

Start FSX. When on the FSX Start Screen select Settings > Controls > Buttons/Keys. Scroll down (or select Radios in Event categary then scroll down) until you find NAV 1/GPS (toggle) , highlight it and click on New Assignment. Create the combination of keys you want to use for switching between NAV and GPS navigation then click OK.

That's it, you are done. It doesn't really have anything to do with being a computer geek. ;)

Mark