Ideally, you should swap the sprocket every time you replace the chain. If you put a new chain on an old sprocket, you’ll wear the new chain out fairly quickly.
Sprockets are available in a range of sizes and usually have a slight dish. This means they sit slightly off to one side when you fit them onto the hub. This allows you to line up the sprocket with the chainring so the chain makes as straight a line as possible between the two, reducing the amount it wears. Take note of which way the old sprocket faces when you remove it so you can get the new one back on correctly. There’s only one common fitting for the sprockets, making them simple to interchange. You’ll have to remove the gear cable and then the wheel to get access to the sprocket. Remove the nut and all washers from the right-hand side of the axle, keeping a note of the order and orientation.
Step 1: The cassette joint sits over the centre of the sprocket and so has to come off first. It’s removed in layers. Correct removal and refitting depend on lining up a series of dots and arrows on the cassette joint. The first layer is a locking ring. It comes off anticlockwise. Steady the cassette joint by holding the cable stop and turn the lockring gently. It only moves about a quarter-turn and then lifts off.
Step 2: The next layer is of yellow dots. Look carefully at the next layer of the cassette joint and you’ll see it’s made up of two parts. Both parts have yellow dots printed on them. If there’s lots of road dirt floating about, you may have to clean it off. Hold the back of the cassette joint steady with the cable stop and then turn the top layer of the cassette joint until all the yellow dots are aligned. It will now simply lift off.
Step 3: Repeat the procedure with the red dots that are now exposed, taking off the last layer of the cassette joint. Lay the parts out in the order they came off, all facing upwards so that you know how they need to be replaced. Wipe all the parts clean as it makes them easier to get back together if there isn’t grit in between the layers.
Step 4: Next is the tricky bit where you’ll need your circlip pliers. Locate the circlip – it’s the ring of metal pressing onto the face of the sprocket. It may be a round section or a square section. It doesn’t quite make a complete circle. Locate the gap, slip the noses of the pliers into the gap and squeeze the pliers to open out the circlip enough so that you can lift it up off the hub. Don’t get in there with your fingers or it will bite you.
Step 5: Before you lift the sprocket off, check which way it faces – it’s not usually flat. Now you can lift the old sprocket off and replace it with a new one, facing the same way as before. Line the three lugs on the sprocket up with the grooves on the hub and ease it into place. Lever the circlip back into place with a small screwdriver, keeping fingers well clear.
Step 6: Replace the first layer of the cassette joint, lining up the red dots and wriggling until it fits snugly. Repeat with the yellow dots. Finally, the lockring fits over the top and then twists to lock in place. It only needs a quarter of a turn or so to lock it, but if it won’t move, don’t force it. Instead, remove and recheck the previous layers fit snugly. Replace washers and nuts and replace the wheel.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Replacing the hub sprocket”