Corliss...
you just have to spread the lip edge a little at the two places they track and pull down from the edges.
They are a crappy arrangement and in need of replacement on most boats our age. With age, the plastic looses its abliltiy to slide, and the metal contacts eventually break.
I replaced all of mine with reasonably priced halogen bulb ones and the better appearance (not to mention the greater light output) is worth it.