Help - Corrosion on expansion tank

I recently noticed that there is some corrosion on the piece above the expansion tank. I don't know if it's called a float valve or pressure release or whatever. The top piece of this, which looked like it was a plastic screw at some point is broken off , so I can't unscrew it. It looks like this piece screws into the top of the expansion tank. I think I need to change it, but I don't know if I can just unscrew it and put a new one on, or if I first need to drain the pipes from the baseboard somehow. There is no shut off nearby, and I'm not sure how to drain the water. I attached some pictures if anyone could help. I think I have a few questions. It looks like the pipe in the last picture just goes down to the floor and is open. Maybe that's where I can let the water out? 1) How do I shut off the boiler? 2) How/do I need to drain the pipes? 3) How do I change the pressure release valve? 4) Do I need to do any maintenance on the expansion tank? 5) How do I fill the pipes back up and make sure there is no air in them?