There’s a lot of conflicting information about this since most of the methods seem to make things worse. There’s a method that recommends the use of baking soda toothpaste to wipe the sunglasses. While both of these may work on light scratches, in reality you actually risk causing more damage. If you have a pair of expensive sunglasses that are otherwise in good condition, you may be able find replacement lenses for them. Ultimately, the best way to prevent scratching your sunglasses is to put them in a protective case when not in use.