Looks like the white tub part should slide right out.

i'd take out the screw on the bottom. Assuming it'll slide out and then the glass can come off.

do you know the manufacturer? Maybe you can find the instruction manual online?

suzanne- That fixture looks so vintage that I'm betting there is no instruction manual online....

I doubt its vintage, especially not based on the tile. I bet its restoration hardware or similar. I would start with that Philips screw at the bottom. (maybe one at the top too) it probably removes the metal trim so the glass can slide out.

Given the tarnishing, I'm still betting on vintage ;-)

But I agree that the screw is the likely answer. (And to the OP: make sure you have a hand on the glass for when the bracket comes loose.)

Before you try anything else, I would try to lift up the top bracket. It might just pull outwards and then the glass could be removed. I have seen several fixtures where the metal brackets securing the glass are spring loaded.
Since you can see a screw underneath, chances are that is the answer but I would try the spring theory first, as it is easy to test.

Hi all! I'm OP! Yes it's the screw at the bottom. Once the screw is removed the bottom metal piece and the plastic cover can both be taken out easily. Thanks all!

Great!