The air filter is in a black plastic canister that is roughly 2 inches in diameter and 4 inches long. Turn it slightly counter clockwise and pull the plastic cover ahead, pry out the filter and replace it with the new canister. It is straight ahead from the carburetor, which is just ahead of the muffler. I wouldn't really worry about the plug gap if it is a big deal. You need to purchase a spark plug gauge to do so.