Changing a tire is easy but it's a skill that takes many attempts to perfect. It can be frustrating and annoying and is a process that takes patience and a cool, bald head. If you go trying to manhandle the thing, chances are you'll end up with bloody knuckles and maybe an ulcer.
Take off you wheel and place it on cardboard or something soft, sprocket side down (unless you like bloody knuckles). Clean up the wheel especially around the valve stem and rim lock. Loosen the rim lock nut and pull the valve core completely out using a valve core remover. You can purchase a valve core remover at CDN Tire.
With all of the air out of the tube, the next step is to break the bead, this may take a little force. Put pressure on one side of the tire with your knees and push down with your hands on the other to get the bead of the tire to separate from the rim. If the bead won't break, you may need to use tire irons.
Using motorcycle tire irons, start at the rim lock and pry the tire over the rim. It is best to have 3 tire irons and work in small increments. Concentrate on one section at a time until you make it back to the rim lock. Be careful not to pinch the tube and BE PATIENT.
Once you get one side of the tire popped off the rim, you can go ahead and pull the tube out. Once the tube is out, check for leaks if you have a flat.
With one side of the tire off, it's easy to get the other side off. Now is a good time to clean the rim and replace the rim strip with 2 layers of duct tape.
Spread some baby powder on the bead of the tire. Work one side of the tire onto the rim the same way you did to pull it off. Next, stuff the tube into the tire. Make sure that the tube has a bit of air and is not twisted up. Insert the valve stem into its hole and thread the lock nut on. (I find this the hardest part)
Similar to how you worked the tire off of the rim, start at the valve stem and work in one direction to the rim lock. Then work in the other direction from the stem to the rim lock. Make sure the bead of the tire is in the center valley of the rim. Work in small increments and be extremely careful not to pinch the tube. You may need to kneel on the tire at times to hold it down.
Once the tire is on, use soap to or a tire lubricant to lubricate the bead and allow it to pop into place. Fill the tire to about 40 pounds of pressure and let it sit for a while to see if it leaks. Tighten the rim lock nut. Note: Make sure that the bead is even all the way around the tire. If it is not, you may need to take the air out and refill to get it right.
It's easier to do once you've seen it done in person. If it's your first time it should take about an hour. Pratice at home with a few refreshments and you will know how to do it on the trail.