Lift the corner of the mower off the ground and place a jack stand under the rear axle. If you cannot lift the corner of the mower off the ground, place a jack under the mower, jack it up, then place it on a jack stand.
Bend the ends of the cotter pin straight with pliers, then pull the cotter pin out of the axle shaft.
Pull the washer off the axle shaft.
Remove the nut that secures the wheel onto the axle shaft with a socket and ratchet if the wheel you plan to remove is on the rear of the mower. Front wheels do not utilize the retaining nut.
Slide the wheel off the axle shaft. If the wheel does not want to come off, spray the mating surfaces of the wheel and axle shaft with penetrating oil, then tap the wheel with a rubber mallet. This breaks the wheel free of the axle shaft.