is it easy to replace the serpentine belt

Jim is right, as always, but the guys at those oil-change joints will always say your serpentine belt needs to be replaced. Give it a good inspection yourself. If there is fraying on the edges, cracks in belt, obvious signs of age them sure, you need a new one. But if it looks good, and has less than, say, 60,000 miles then they are just pushing something you don't need.

Dont trust the sticker under the hood, Draw yourself a picture before you start of where it goes exactly, and if you have any doubts about the condition of the belt then do it. Preventing problems is always a good plan