if the lens is broken then fold it shut then pull forward on the top edge to look behind it. if its like the ones I remember it has 3 or 4 clips that go in around a center pivot ball. it should un clip form the pivot ball when pulled gently forward from the top. I would be very careful as not to break any plastic tabs off.
on a side note you may just try and find a replacement lens. check pep boys or one the big chain stores for a stick on lens that doesn't require you removing the old lens