Move the tractor to a flat, level surface, park it and shut it off. Pull the hand brake and wedge stones under the other wheels to ensure that they do not accidentally start to move.
Place a jack under the tractor, consulting the maintenance manual to for location. Lawn tractors are typically designed with a space on the chassis where you can safely insert a jack. A regular car jack will do. Jack up either the left or right side of the tractor, whichever side has the rusted wheel. This enables you to more easily access that wheel and remove it without having the weight of the tractor bearing down on you.
Dry-scrub the rusted bolt and nut with a wire brush to remove as much of the rust as you can.
Spray down the nut and bolt with a generous amount of thread loosener such as spray oil. Let it soak in before proceeding.
Wrap your wrench around the nut and rotate left. If it resists, spray it down with the lubricant again -- you may even have to let it soak overnight. Continue this process until the nut starts to give. Repeat with other rusted nut until they are all removed, then pull off the wheel.