rusty

Need some help. I am thinking of adding breakers to the the old, rusty panel, and but not sure how to open it without causing damage. Some pictures of the box below There is a knob near the center bottom, both does not move easily. Thanks in advance!

Based on the various guides on this website, I'm probably looking at replacing my water heater. But this is a last ditch effort to do some DIY before calling a repair tech or dropping hundreds on a new tank. One month ago the lower element starting leaking rusty looking water. So I replaced the...