You can't change the GPU in a laptop. You will need to replace the mobo for it(hard to get). What happened? The display is black, or GPU is artifacting?
It's my personal experience,
See my signature for specs, I have a Compaq 621.
Had a thick line across the screen for a few months, then display went blank one day in March. Shop guy said GPU has gone bad so the mobo needs to be replaced.