how to replace a water pump on a 4.2

I don't think it's something you really want to do. It's about an 8 hour job for a trained tech, require more than just basic skills. The timing chain is involved, the torque specs are very important, and it has to be followed. If you think you can handle this I'd suggest to get a repair manual, read and understand the water pump replacement and related procedures few times before you will start on the job. I think it would be the best to leave it for a Chevy tech. Zee