when I change the serpentine belt started slipping

Hi:
It sounds like the belt tensioner is bad, and when you removed the old one, it finally went. Check to see if the belt is tight or if it seems somewhat loose. This guide can help you change the belt out.

https://www.2carpros.com/articles/replace-serpentine-belt

and

https://www.2carpros.com/diagrams/chevrolet/silverado/1999



Check out the diagrams (Below). Please let us know what happens.