re: Changing an alternator out on a truck.

As stated, definitely disconnect the battery first.
May want to consider swapping out the belt and belt tensioner, it there are a lot of miles on them.
If the alternator is near the top it should be easier than if it's mounted low on the engine. If it's low, you may need to remove plastic shields below or at the fender well. They protect the alt from water and are usually held on with plastic clips.