How do I replace the serpentine belt on cummings 350 isc

Assume you have verified that you have to correct belt.

Most belts with automatic belt tensioners (usually 1/2" square drive) are pretty tuff to rotate. But, you should be able to do it-- even if you have to use a short pipe/cheater bar.

Now, if this is a rear radiator, access is a lot more difficult and goes much better with someone below and someone above.