i have inherited a greenhouse and all the glass takes out except the windows which are pot riveted to the roof joist, i have a broken pane, how do i replace it when it looks like its sealed

I've never heard of glass being pop rivited before. All my glass is held by removable clips.
When I first got my greenhouse there was the remains of a pane of glass that had been sealed in place with silicon.
Using welding gloves for protection, a scraper and great care ,I managed to carefully remove the broked bits and the silicon.
Sorry I can't be of more help, I hope you manage to resolve this problem.

Is the makers name on the green house might help?

you do not say if the greenhouse is mettal or wood , I canot see how glass can be pop riverted plastic may be ? then when you say fully fited windowes it makes me think of wood were the glass slides in to grooves and is held by a pice of wood on the bottom( glass block ,pop rivets are easly removed just cut the head off, with a little more information we may be able to help you