This Youtube video shows step-by-step how to upgrade your stock Fiat 500 gear knob. The glue in the original gear knob is quite strong stuff. Heating it might help to remove the cap.
The same quide with fotos: (in Finnish only)
http://www.500f.net/blog/category/vaihdekepin-nuppi
The same quide with fotos: (in Finnish only)
http://www.500f.net/blog/category/vaihdekepin-nuppi