Have recently done this, the glass is held on with a putty type substance albeit VERY well stuck. I found the glass can easily chip if you lever against it....
I took a stanley knife & carefully pryed away between the glass & plastic removing as much of the putty as possible then got a flat edged screwdriver & worked it in between, making sure that any 'leverage' was on the plastic. once it was in I carefully worked around the whole light until the glass came free.
Hope that helps!