Locate the graphics card inside your computer (if you do not know where that is, consult the Opening A Computer and Parts Identification video in the Beginner section)
Unscrew the graphics card from the motherboard
If it is a newer graphics card, unplug the extra power from the top
Finally, you might have a clip on the bottom of the graphics card that you must push in order for it to come out
Replace the graphics card (clip it in, screw it in and add extra power if needed)