On some newer, high-end notebooks, the graphics card can be accessed simply by opening a panel on the bottom of the computer. But for most older machines, disassembly will take more steps.
The first thing you need to know is that your laptop can probably support only a handful of very specific graphics cards, if the card can be upgraded at all. Your best source for an upgrade card will likely be the manufacturer that made your portable.
Be sure to consult your laptop's service manual (which can usually be downloaded from the manufacturer's support page) before attempting to open up your machine. Also, make sure you guard your hardware against possible electrostatic discharge by wearing an antistatic wrist strap whenever you handle any internal components.
Laptop designs vary, but the upgrade process usually begins with removing hinge covers: Pry up any plastic hinge cover and pull it away from the chassis. Avoid excessive force, as pulling too hard can break the plastic parts.