Directions to Open Tellabs Battery Backup Unit?

My ONT won't stay powered after a recent power outage. The backup battery has needed to be replaced for a while now, but that hasn't affected my service. I've done some research, and I believe I need to disconnect and/or replace my backup battery. However, I cannot get the panel open to access the battery.

I'm able to get the bottom left corner open, however, it is sticking at the top, as if there is a screw holding it in place. Unfortunately, because of where it's located, I cannot get a clear view of the top of the box.

I'd appreciate directions on how to get it open and/or a picture of the top of the same model, so I can try to figure out how to open it without breaking the panel.

Thanks in advance for your help!!

Here's a picture of the box: