How is the serpentine belt replaced on a 2000 Astro? What I thought was the tensioner appears to be bolted in place.

The belt tensioner is not a manual tensioner, it is automatic it. Use a stick or long pole to push the pulley on the tensioner backwards, while pulling the belt off. Same way to put it on. After the belt is inplace, push the tensioner back in place and it automatically tensions the belt. Hope this works.