The large round or square thing at the top of the engine is the air cleaner housing that contains the air filter.

Unscrew the wing nut at the top or unclip the clips, remove the top cover, and simply lift the round filter out.

The filter is a pleated paper device and you should be able to see light through it.

If no light comes through, hold it in its normal, bottom-down, and tap it gently on the ground a couple of times in order to remove any dust.

If it’s still not possible to see light come through, replace the filter.