1998 Chrysler Cirrus How to change the Alternator 1998 Chry

While my car is running or not the car light dims anytime I do something such as unlock the doors. The little battery light comes on sometimes but other times it doesnt. I believe it's the alternator but I'm not sure how to change it. Any help will be much appreciated.

Also I took it to Adavance to see if it's the battery or Alternator and he said that the low batter out put shouldn't even crank the car, but I have never had a problem starting the car it cranks right up.