there's a gun upstairs that can be used to break the glass, it's on a table. Also there's a key for the backdoor behind the jewelry store by some trash bags to avoid the lasers that trigger the alarm at the front door.

Also kinda off-topic, I just noticed in Sneak Thief 1.0 that the Jewelry Store on the level selection is misspelled lol. It says "Jewlery Store" instead of jewelry.

And of course, mojojo is here to help the community.