How to open this clock?

[MaineT]

I know a lot of you are Clock collectors. I found this one today with a pharmacy theme and I was kind of thinking everybody seems change out all the lenses. It doesn't have a ring and seems sealed. Everyone saying you have to watch out with the Pam clocks because they could have fake lenses, I'm okay with the repro lens because I don't like  this theme....  How does it open?

I don't want to break the glass lens.

I also put a pic of this weeks sign finds. The round one is marked 1930.