If you are talking about cleaning the part in the red square, just unscrew it and pull it out. Should be an 18mm socket.
That's the part that makes the spark that ignites the air/fuel mixture. It can be cleaned in a small variety of ways. I've used some fine emery cloth followed by a shot of brake parts cleaner to remove residue. Careful of your eyes. And wear gloves.
Be careful threading the spark plugs back in. Don't cross-thread them. Very bad.
Do it by hand, or with just the socket, and possibly an extension, on them and no handle. If they thread in that way with no resistance they can then be tightened up with the handle. If you don't have a torque wrench, tighten them until they seat, and then about another eighth of a turn. But get a torque wrench.
Did I say to get a torque wrench?