How do you take off the glass of a Pinball machine?

I have a Bally pinball machine from somewhere between 1975-1979 (I don’t remember) I have never really used it, I just got it from my uncle a few years back and it’s been sitting in the basement.

It is suppose to work, but there is a piece of cardboard stuck where the ball is suppose to be.
(I’m assuming he used it to keep kids from playing with it because he said that the ball hits the glass for some reason and makes a loud noise (which is another thing that needs to be fixed))

But we aren’t sure how to get the glass cover off so we can
see what all needs to be fixed.

There is a number to call for maintenance but of course.. it’s ancient =] It doesn’t even have an area code, so if we DO have to have a professional come out and fix it… who does stuff like this?