It's just snapped on. Orange lens pops out of the black frame. Maybe a tiny flat screwdriver to help pry it free. I've lost the front clear one that is a bit larger several times. Finally replaced the unit with a super bright led one off amazon. http://a.co/ilOwUnZ
Replace with LED lamp, should be able to get one for about $12.
Purchased one for ours after the stock lamp burned out just after a month, I keep ours on all the time while the trailer is stored in our driveway.
Over a year and still going with the LED.