How do you get the casing around the fan off so you can change the belt?
Or do you have to unbolt the fan blades first?

 

You don't have to remove the shroud to replace the belt. You might have to crawl under the truck to get the belt started on the lower pulleys. And yes, to remove the shroud you'll have to remove the cooling fan and take them both out at the same time.

 

Very carefully...sorry, couldn't help it.

 

when you are facing the engine, there is a damper pulley that is on a spring loaded arm on the left side of the fan. You need to relieve the pressure by using a long armed rachet or belt removal tool and swing the arm up (or down, I can't remember ) then you can just slip the belt off the other pulleys