Is more then necessary to have your lawnmower working properly, especially if you want to save some time on lawn maintenance.
One thing that you can do to tune it up is change the motor oil. The machine should have a drain plug underneath it.
Also,you can check the spark plug. Just remove the spark plug wire and use a socket or ratchet to remove them. Turn them to the left to loosen them. To buy some replacements, just read the code on the side of the spark plug. Then thread them by hand to avoid cross-threading.
Changing the air filter will also help your lawnmower. Loosen the housing cover bolts and then remove the old filter. Clean the interior of the housing. Install the new filter.
Blades are maybe the most important components. Sharpen them. Also replace old gas, mufflers and wheels.