Look closely at the wood trim around the inside of the glass, it should pop out. It is probably being held in with either staples or small nails. look closly and try carefully prying it loose.

Welcome gracie,

You wouldn't mean MUNTIN, would you? I think MUTTON refers to: the flesh of a mature sheep used for food

Sorry, couldn't resist! (Hopefully the other post was a bit more useful to you!, good luck!)