Park the riding lawn tractor on level pavement. Shut the mower off and engage the parking brake.
Place a jack underneath the mower near the flat tire. Make sure that the bottom of the frame of the mower is securely placed on the top of the jack so that it does not slip off when you raise the mower,
Free the rim from the axle or spindle of the riding mower. The method, and the tools that you use will largely depend on the type of riding lawn tractor that you are using and whether it is the front or the back tire that needs changing. To remove the center cap, pull it off of the wheel hub with a pair of pliers. To remove axle pins, pull them out (with the help of pliers if need be). Remove a C-clip by grasping the closed side with needle-nose pliers and then pulling it off. Remove lug nuts by unscrewing them with a lug wrench. Remove washers by pulling them off by hand. If you are unsure of the parts on your riding lawn tractor's tire or the tools you will need to remove them, call the manufacturer's customer support line.
Pull the rim off of the axle and then set it on the ground with the outside of the rim facing upwards.
Release the remaining air from the tire. Locate the air valve and press down on it until all of the air leaks out. To speed the process, use a valve stem to remove the valve completely.
Lay the tire flat on the ground with the outside of the rim facing up.
Remove the tire from the rim. Depending on the age of the tractor, this may be your toughest job. The "bead," or the portion of the lawn tractor tire in contact with the rim, often becomes stuck to the inside of the rim. An effective way to remove the tire is to insert a flat-head screwdriver between the rim and the tire to break the seal between the bead of the tire and the rim on both the inside and outside of the rim first. Then use the flat-head screwdriver to help you pry the top of the tire off of the rim first. Then pull the rest of the tire off of the rim. Stubborn tires may be coaxed along with a bit of tire lube.
Put the un-inflated new tire onto the rim. To facilitate the process, wet both of the beads of the new tire with tire lube. Then position the tire over the top of the rim at a 45-degree angle. Push the lower side of the tire over the rim (if this is difficult, wedge the screwdriver tip under the lip of the rim and push down on the tire, taking care not to damage the tire). Then work around the circumference of the tire, pushing it down over the rim section by section.
Replace the rim and tire back on the axle. Tighten any lug nuts with your lug wrench, replace any washers, squeeze the C-clip back on with the pliers, and rap the center cap back on with a rubber mallet.
Secure a ratchet strap around the new tire and tighten it until the inside of the tire rests on the inside of the rim's lip.
Lower the mower via the jack until the tire is just touching the ground.
Fill the tire with air using an air compressor to fill the tire to roughly half of the manufacturer-prescribed PSI. If you have removed the valve stem, put it back on first.
Remove the ratchet strap.
Fill the tire to the manufacturer-prescribed psi.
Lower the mower all the way down and remove the jack.