make sure there's nothing plastic around the area on both sides of the board and then just heat up the general area of the graphics chip, you have to get it pretty hot, keep an eye on the little components on top of the GPU and around it and heat it until the solder on those just turns shiny and then leave it to cool for a few minutes before moving it (don't blow it to cool it down, I've seen somebody blow the surface mount caps right off the board doing that :))
Also if it's got a sort of stick on heat transfer pad on top of the GPU it's worth finding a 1mm thick bit of copper, cutting a square the same size as the top of the GPU and replacing the pad with the copper square (use a decent thermal paste on both sides of the copper)