The graphics chip is integrated into the motherboard, so no, you cannot simply upgrade just the graphics chip. However, this laptop came with two available graphics cards: a Radeon HD 6770m, and a Radeon 7850m. If you have the 6770m, you can upgrade to the 7850 model by replacing the entire motherboard. As this is replacing most of the major components along with the card, it could be quite costly, as much as a new PC.
Here's the service manual:
http://h10032.www1.hp.com/ctg/Manual/c03317166