Slide the jack under the snowblower's frame. The most secure place to lift from is under the motor. Jack up the snowblower until the tire can easily go onto the machine.
Unscrew the original tire's bolt in the middle of the rim with a 3/8 inch wrench. Slide the bolt off the arm, and place it on the ground.
Pull the tire off the arm. It should come off with no problem.
Pry the old tire off the rim with a flat screwdriver. The tire might feel like it won't come off, but it will.
Fit the new tire on the rim.
Take the new tire with rim, and slide it on the arm in place of the old tire.
Tighten the bolt back on the arm so the tire is secure.
Lower the jack slowly. You don't want the snowblower to slam to the ground.