It really depends on your laptop and the options can be very limited. For example, my old laptop came with a nVidia Geforce 9500M and the best it could install 9650M or something like that, basically the minute difference wouldn't have been worth the money. My current laptop, with Intel Integrated graphics cannot be upgraded, or so I have been told. Plus the process isn't necessarily that easy in comparison to a desktop and you are risking damaging your laptop. It's probably something best left to a professional.
My best advice would be to consult your laptop manufacturer with as much information as possible, including specific information about what model you own. You never know your manufacturer might have a service centre who can offer you the service for a price. But I think if anybody is an expert on what your laptop can handle and how it can be upgraded would be those who made it.
PS. Check the conditions on your warranty, some can be made void if you make any alterations on the hardware.