GM-GMC: fuel pump chevy pickup, chevrolet pickup, chevy pickup

Question
how to remove and replace fuel pump 1990  chevrolet pickup 305 motor, 1500 series

Answer
You need to either drop the fuel tank and remove the sending unit to replace the pump or remove the box and remove the sending unit from the tank and remove and replace the pump that way.  It can be a difficult job on a vehicle that age.  Lines generally are heavily rusted and difficult to remove.
That is a pretty vague description but if you get the tank out and have any more questions post again.
hope this helps