Normal wear and tear on a bike will eventually require replacement of the sprockets. Over time as you ride your bike, your chain wears down the teeth on the rear sprockets; the smaller sprockets tend to wear quicker than the larger ones. You may need to replace the entire cassette or just one or two of the worn sprockets. It's relatively easy to remove rear sprockets for replacement.
Remove the rear brake. Squeeze the rear brake with one hand and simultaneously disconnect the cable using your other hand.
Remove the back wheel. Most bikes are equipped with a quick-release lever for this purpose. If your bike has one, move the lever into the open position to release the wheel. If your bike is not equipped with one, use a crescent wrench to remove the nut that holds the wheel in place.
Place the wheel on a flat surface with the sprockets facing up. Wrap the chain whip tool around the largest sprocket and secure it. The chain whip tool holds the cassette in place so that you can loosen the lockring.
Place the sprocket removal tool around the lockring. Use a crescent wrench to turn the lockring counter-clockwise, so that you can loosen and remove it.
Lift the sprockets and spacers from the wheel hub and place them on your workbench in the same order that you removed them. You are now ready to replace the worn one.