TOPIC: replacing plexy windows on a catalina

I hope I'm in the right section! I'm replacing the side windows in a 67 Dorsett Catalina and I'm sure one of the experts can point me in the right direction. I tried pulling the inside rim off with no luck!!! Does the whole window have to come out?? I've already snapped one piece of plexy and that stuff is pricey!!! PLEASE HELP!!!!