Sturmey Archer is best known for its three-speed internal-gear hubs. These hubs allow you the versatility required to climb moderate hills without the derailleur mechanism and sprocket cluster that accompany most road bicycles. Because three-speed hubs typically come with only one sprocket, it tends to wear out faster than several sprockets in a cluster. Depending on your experience and preference, you may wish to swap out a sprocket for one of a different size.
Remove the rear wheel from your bicycle by loosening the nuts with a spanner wrench. If your real wheel has a quick-release, you can remove the wheel with your hands. Take the chain off the sprocket and let it hang in the rear triangle of the bicycle.
Place the wheel on its side with the sprocket facing upward. Take a careful look at the mechanism and locate the spring circlip. This is the snap ring that holds the sprocket in place on the hub.
Fit the flathead screwdriver into the circlip's groove. Use leverage to pry up the ring a bit at a time, gradually moving around the outside groove. Set the circlip aside.
Remove the sprocket. Once the circlip has been removed, the sprocket should come off freely.
Replace the old sprocket. Make sure that the circlip is snapped back into place and made snug by tapping it lightly with a hammer and punch tool.
Replace the wheel. Thread the chain over the new sprocket, clean up and take a test ride. Make any necessary adjustments.
Adding a bit of lubricating oil to the sprocket and circlip will make this process easier if the bicycle is old.
Make sure that the circlip is seated correctly and snug against the new sprocket before putting your wheel back on. A loose circlip may cause the sprocket and hub to fail.