how do you get the glass out?

You'll need a stout pin or a fine jeweller's screwdriver. There's usually a tiny notch in the internal bezel ring. Lever the tip gently into that notch and ease the ring upward. The glass sits loosely underneath this metal retaining rim.

My grandmother left me a gold locket and it has some old dark hair inside, should I take the hair out to clean it?

Most dealers suggest leaving original hairwork just as you found it. That woven hair forms part of the specific social history of the piece. If you try to clean inside the compartment, you risk dampening the hair. Moisture causes historic hair arrangements to expand and warp.

what does back and front mean?

That stamp indicates a specific construction method often used in the late nineteenth and early twentieth centuries. The flat front and back plates are formed from solid gold, but the concealed hinges and internal frames are base metal. This technique was a common trade practice in large Birmingham workshops. The base metal core adds structural rigidity while keeping the overall weight down for the wearer.

I bought one online but it keeps popping open while I am walking around, is there a way to fix the clasp so it stays shut?

A jeweller can usually adjust the snap closure. Sometimes the metal edge just needs a fractional bend inward to restore the friction catch. It's a quick repair at the bench.