It is highly unlikely that you would be able to upgrade the GPU. Most laptops have the chips welded to the motherboard and cannot be removed. The only company I know of that has removable GPUs is Alienware.
Overclocking on laptops is not really a good idea since the whole thing is cooled by just one fan and doesn't have good airflow. It probably won't boost the performance more than 10% anyway. Try at own risk.