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For a Schrader (American) style valve, use a small tool (like a thin Allen wrench) to depress the plunger inside the threaded cylinder. For a Presta valve, unscrew the top part of the stem to release the air. With a Dunlop valve, loosen the cap a few turns and then pull on the valve tip.  Schrader valves are the same type found on car tires. Presta valves are thinner and longer than Schraders, and have a locknut at the tip. Dunlop valves are thinner than Schraders and thicker than Prestas, and threaded only near the top. If your wheel has a lock ring that screws onto the valve stem to hold it to the bike rim, remove it after deflating the tube — but don’t lose it! If possible, avoid metal tools and use a plastic tire lever instead. Feed one lever between the outer tire and wheel rim, and pop out a section of tire — instead of sitting in a channel on the inside of the rim, it should now be off the rim in this spot. Leave this tire lever in place.  To reduce the risk of damage to your wheel frame, buy a set of inexpensive bike tire levers for the job — you can get them at any bike shop or online. A pair of spoon handles or flathead screwdrivers will work as levers, but you have to insert and pry with them carefully so you don’t scratch up or bend your wheel frame. Stick the other tire lever into the gap between the rim and tire created by the first lever (which should still be in place). Slide this second lever all the way around the rim, and the outer tire should pop out of the channel as if you’re unzipping your coat. Reach into the opening you’ve created with the levers and grab onto the rubber tube inside. Work your way around the wheel and pull it out completely. When you get to the valve stem, push it down through the rim and pull it free with the tube.
Deflate the tire fully while it’s still on the removed wheel. Pry out a section of the outer tire with two simple levers. Pop out the rest of the tire from the wheel rim. Pull the tube out from between the outer tire and wheel rim.