Turn the mower off and disconnect the spark plug wire boot from the mower.
Turn the lawn mower upside down.
Loosen the lock nut with an adjustable wrench, turning the nut counterclockwise.
Remove the shoulder screw, which will cause the old tire and the washer between the tire and the mower to come off.
Place the washer in place over the hole -- the front tires have three holes to set the tires at different heights; use the same height on both sides. Line up the new tire with the washer so that the hole is open. Push the shoulder screw through the tire, washer and mower body.
Replace the lock nut on the end of the shoulder screw. Tighten it with the wrench.