By David Fiedler, About.com Guide
The first step is to take the wheel with the flat off your bike. Do this by loosening the nuts or quick release mechanism that holds the wheel until it slides out. You may need to loosen your brakes to get the wheel off. These often have a quick release mechanism too. If you are removing a rear wheel, it will also have to be lifted clear of the chain.
Using tire levers, remove the tire by wedging the tire tool between the tire and rim, and then prying upward to lift the tire away from the rim.
Keeping the first tool in place under the tire, repeat this step about four inches away with the second tool to pull more of the tire over and off the rim. Repeat this step as you work your way around the rim. The one edge of the tire you’ve been working on should start to come free of the rim quite easily. You can finish this step by simply sliding the lever underneath the tire the rest of the way around the rim.
Next, you’ll need to remove the valve stem from the rim. This is the metal valve that pokes through the rim used to inflate the tube. Coming from the center of the wheel by the spokes, locate the valve stem and push it up and through the hole in the rim so that it no longer protrudes through the rim.
Remove the tire and tube the rest of the way. You can usually do this easily by hand, but if you have trouble getting the edge of the tire completely over and off the rim you can use the tire levers again. Once the tire is off, pull the old tube out of the tire. You can then either discard the old tube or attempt to patch it.
Take the new tube and with it still flat, work it into the tire, laying it where it goes when it is inflated on the rim. Take care that the tube is not crimped or twisted at any point, and that the valve stem points to the center. Some people find that the tube is easier to work with if you put just a little bit of air in it, enough to hold it in the tire.
If your tire is flat because of a puncture that came from hitting a road hazard, after you remove the old tube check the inside of the tire thoroughly to make sure whatever caused the flat — glass, a piece of metal, etc. — is not still lodged in the tire to play a nasty trick on you and pop a second tube.
Put the tire and new tube back on the rim by first lining up the valve stem with the hole that it will need to go through on the rim. This is the reverse of what you did in Step 4. You do this by working the first edge of the tire back onto the rim, easing the tube onto the rim at the same time. As you seat the first edge of the tire onto the rim, use your fingers to carefully guide the valve stem back into its hole. Finish putting the first edge of the tire completely on the rim.
Use your hands to work as much of the second edge of the tire onto the rim as you can. It will become more difficult as you go, and finally you will need to use the tire levers to put the last part of the tire onto the rim. Do this by wedging the tire tools against the rim below the edge of the tire that still needs to go on, and then working one lever and then another to bring the edge over the rim until the whole tire is seated snugly and comfortably once again inside the rim.
Once the new tube and tire are back on the rim, do a quick check with your eyes and fingers around both sides of the rim to make sure that the complete edge of the tire is inside the rim, and that at no point is the inner tube pinched between the tire and rim or protruding over the rim.
When you reinsert the valve stem of the new tube into the rim, be sure that it comes straight out of the hole and not be angled in any direction. Any tilt in the valve stem tells you that the tube is not centered over the hole. You can correct this by sliding the tube and tire around the rim just a bit in the proper direction to correct the tilt.
Using the pump, inflate the tire to the pressure recommended on the sidewall. (Related article: Keeping Your Tires Inflated to Proper Pressure – How Glamourous
As you put air into the new tube, be sure the tire is filling consistently. Any uneven inflation that you notice, such as a bubble or highly inflated portion of the tire while another part remains flat, tells you that your tube is pinched or twisted inside the tire and needs to be reset. Correct this by letting the air out of the tube and repeating Step Two, which allows you to look for the spot that is pinched or twisted. Many times you can fix this without removing the tire completely again. Replace the tire as you did in Step 7 and try inflating the tire again.
Put the wheel back on your bicycle, reattaching the nuts or quick release mechanism and resetting the brakes and replacing the chain as necessary. Check to make sure that the wheel is aligned properly, that it is held securely and that it spins cleanly.
(c) David Fiedler, licensed to About.com