A hammer... Or a glass cutter.
Nominally, you are supposed to be able to remove the black plastic part that goes around the glass without breaking it. Plastic which is 25-30 years old, and has probably been sitting outside exposed to the weather for most of that time. So how lucky do you feel??
It is probably easier to break the glass (or cut it) and then fish out the plastic. That is, if you're replacing the glass anyway.
--DD