How do I replace the water pump in a 93 4.9 l Cad Deville?

That's a pretty involved procedure. Your best bet is to get a copy of the manufacturer's service manual, then read through the pages of instructions with line drawings to see if you want to pursue it. There will also be a section that shows any special tools that might be needed.

We don't have a video for it but here are the instructions and torque specs needed to do the job. It says it takes a little over two hours to do so be ready. Check out the images (below). Let us know if you need anything else.