How do you change serpentine belt on 1983 S10?

It depends on whether you have the self-adjuster or not. Without the adjuster the steering pump needs to be loosened at the three bolts that hold the pump to the bracket. The bottom bolt is the tension adjuster. With the self-adjuster, you need to rotate the adjuster towards the engine to slip the belt off. Once the belt is off the tensioner it will move out of the way. Be prepared to use a block of some type to hold the tensioner so it won't hit the battery case. The nut on the tensioner is a 15mm size

Also, once you have the belt off, be very sure you put the new one on the right way. There should be a sticker on the engine cowl that shows you how the serpentine belt should route around the engine pulleys. Once you have it routed right, make sure you tighten everything very tight, making sure you have put enough tension on the belt. This is usually a tough part of the process because you need to get the tension against the adjusters, which keep the belt tight as it moves