How to open & change the battery?

I acquired a Vintage (1970's) Staiger West German Quartz Clock in French Crystal Casing from my mother-in-law. It is lovely, but the back of the clock insert is solid and I can't figure out how to get the top casing off to change the battery or set the time. It has 3 hinge things on the side but I can't see how to release them.

Maddie SC
May 2015