Unfortunately, you cannot upgrade just the GPU. The GPU is soldered to the motherboard. To upgrade, you would need to remove your motherboard and replace it with a motherboard containing your desired GPU, such as these from Parts-People. Also, if your current motherboard has only integrated graphics, you would also need to buy the necessary heat sink.
I Love Computing and am upgrading my dell inspiron 15r 5537, I recently went from 6 to 16gb ddr 1600MHZ of ram. I am wondering if i can upgrade my integrated (or dedicated) gpu to a NVIDIA GeForce 470 GTX. 
Dell Inspiron 15R 5537