The most common reasons for replacement are damage and discoloration due to oxidation. Replacing the lens on a headlight is one of the easiest auto repair tasks to do and requires nothing more than a screwdriver. The most important part of replacing a lens is to get the correct size lens for the vehicle.
How do you separate headlight lenses from housing?
https://www.youtube.com/watch?v=anN0vYBj6Uo
How do you change a headlight on a 2007 Volvo xc90?