The vast majority (like 99%+) of laptops do not allow you upgrade the graphics card. Basically only certain gaming oriented laptops ($1500+) may allow you upgrade the graphics card. You'll need to find out 1st before buying.
Also, it is very difficult to buy a laptop graphics card since they are only sold to system builders like Dell and HP. You'll need to rely on people selling them through eBay which may be used graphic cards, or otherwise obtained, maybe in a "shady" way.