Although I've never done so personally with any of my Lynx II's, my understanding is that you can remove the lens from the outside, although it apparently takes some force to do it. Here's a quote from another thread on Atari Age:
According to John
Daniels, "The front screen on the Lynx 2 snaps on and off (easier to snap
on than off though). The transparent area and the area surrounding the
small buttons near the screen is one big piece of plastic. It snaps on
with a lip on the top and bottom edge. ... It takes a pretty sturdy sharp
edge, but once you start it moving, it just pops off and then you can
clean the area between the plastic screen and the LCD."
For cleaning, plain water or cleaning liquid will work fine. It is
recommended that you allow your screen to dry before reassembling the
Lynx, to reduce the danger of damaging the electronics."
Hope this helps.