Place the hand truck flat on the floor with the wheels facing up. The package skid and the handle will be on the floor.
Straighten the bent portion of the cotter pin on the outside of the defective tire. The cotter pin secures the tire to the axle and there are washers between the cotter pin and the tire hub. Pull the cotter pin out of the axle with the needle-nose pliers.
Pull the flat washers off of the axle and pull the defective tire straight off of the axle. Place the center hub of the replacement tire over the axle and push the tire against the flat washers between the hand truck frame and the tire hub.
Place the flat washers back on the axle and against the tire hub. Replacement hubs can be narrower or wider than the original tire hub. Locate the hole in the axle for the cotter pin. If you cannot see the hole, then you will have to remove a washer. If there is a significant space between the hole and the washers, then you will need to add additional washers. The cotter pin should rest against the washer to keep the tire straight.
Insert the cotter pin through the hole in the axle and bend one side of the cotter pin around the axle with the needle nose pliers. Stand the hand truck back up on the wheels and package skid.