How Do I Change Fuel Pump On 1991 Chevy. 1500 Truck?

That's really a tough problem and it is not possible that I can teach you all the steps for doing that. But I'm giving you a very good link and by going through that link you'd learn how to change the fuel pump. The link is given below
click here

I would slide the box back about two or three feet. Then you can see the top of the gas tank and remove the fuel pump easily. The box if I remember has eight (8) bolts holding it in place. An impact wrench would help. After the bolts are removed you need to undo the wiring harness going to your tailights. You need to get a couple of friends and something like a wooden horse to support the box when you slide it back. If you've ever had to drop a gas tank, which is the other method you'll agree that sliding the box back is much easier.
Jaker