It's soldered on to the motherboard, there are two ways to do:
1. Unsolderd the BGA package (needs a BGA unsolder equipment that costs only 20-30k USD, then you add solder paste to each one of the 400+ balls and then you buy the same package new ATI chip (that is pin-to-pin compatible) and then add massive heat to put on the chip. Even in notebook factories this procedure is only 25% successful since removal of BGA usually destroy the board, any reinsert of a BGA is also highly likely to go wrong.
If you however succeed, then you need to beg HP for a BIOS upgrade for your new VGA chip. It's very unlikely they will do that unless you bribe the BIOS engineer USD5000-10000.
2. The cheapest possiblity is to buy a new notebook
Notebooks are very unflexible on upgrades. ASUS used to have one notebook with upgradable VGA board, but only one version of VGA was available... later they scrapped this idea due to too many problems.
Usually HDD, CD, RAM and CPU are upgradable, but each notebook is different and in many cases only RAM is upgradable.
Lyngemark