Replacing the air cleaner element is a simple, routine maintenance operation. You should be careful, however, to keep dust and dirt out of the air cleaner housing, as they accelerate engine wear. If the outside of the air cleaner housing is dusty, wipe it with a clean rag before beginning work.
On carburetor equipped engines, you typically have to remove only the top cover of the air cleaner. Remove the wingnut at the center of the housing and then unclip the three clips situated around the sides. Then, pull the top cover off and remove the air cleaner element. When installing the new element, make sure it seats squarely around the bulge in the center of the lower air cleaner housing. Install the housing top, turning it until the wingnut mounting stud lines up with the hole in the top (it's usually off center). Note that the top cover should seat tightly all around. Install the wingnut and reclip the clips.