The lens is sealed in place. It is possible to remove it, but it's not easy - you have to remove the electronics and all wiring, then heat the assembly to soften the sealant. And of course re-seal it afterward. Guys that used to upgrade to projectors, or add halos or Angel Eyes would do this and have instructions for specific models (i.e. what temperature to bake them at). My advice is to first try removing the bulbs and using compressed air, vacuum, and pipe cleaners to dislodge and remove it. If that doesn't work, you might have better luck removing it from the car to try the same cleaning. Removing the lens would be the last resort.
EDIT: well, there you go - Glyn has the disassmbly instructions linked!