The rear wheel cassette on your bike contains a cluster of cogs that allow for different gear ratios. The cassette is found at the centre of the rear wheel, attached to a freehub, which allows the cassette to rotate independently of the wheel. To replace the rear cassette, you must remove the old one.
Ensure that you have a proper cassette lockring removal tool. They come in many shapes and sizes. Check with Parktool.com to find a cassette lockring tool that suits the design of your bike's cassette.
Remove your rear bike wheel. Shift the chain to the smallest rear cog. Unlock the skewer securing the wheel to the bike. The skewer will be found running through the centre of the wheel. The wheel can now be removed.
Place the wheel on the ground between your legs, with the cassette facing away from you. Fit your lockring tool into the cassette. Fit a drive ratchet or adjustable wrench to the lockring tool.
Wrap the chain section of your chain whip over the largest cog on the cassette. The handle of the chain whip will be positioned at the two o'clock position, while the handle of your wrench will be at the opposite, 10 o'clock position. Push the two handles down. This will loosen the cassette lockring, allowing for removal of the cassette.
Fit the new cassette onto the free hub. There are a set of splines on the freehub that keep the cassette in place. Align your cassette with these splines. Once aligned, the cassette will fit comfortably onto the freehub.
Tighten the cassette lockring. Fit your cassette lockring removal tool into the cassette. Turn the tool clockwise with a drive ratchet or wrench.
Check the cassette for play. There should be no lateral movement from the cassette. If there is movement, the cassette lockring will need to be tightened further.