Instead of using an older style freewheel for the rear sprockets on a multi-speed bike, many bikes today use a special cassette. Rather than mounting the freewheel directly to the hub, this design uses a spline permanently mounted to the hub body to which the cassette is attached. This allows you to more quickly change out sprocket sets. Removing the cassette requires a special tool called a chain whip, which consists of a length of bicycle chain attached to a metal handle. The chain whip stabilises the cassette, allowing the removal of the spline lock ring, which holds the cassette in place.
Shift the drive chain to the smallest sprocket on the rear gears. Remove the axle nuts from the axle or open the quick release mechanism. Remove the rear wheel from the bike frame. Unthread the cone nut from the quick release mechanism and remove the quick release skewer from within the hub.
Set the length of bicycle chain on the chain whip onto the biggest sprocket on the cassette. Insert the pin on the spline lock ring tool into the hole in the axle and down onto the spline lock ring in the centre of the cassette. Align the notches in the tool with those in the lock ring.
Fit the socket spanner onto the end of the spline lock ring tool. Grip the handle of the chain whip with one hand and hold it still while turning the socket spanner counterclockwise to unscrew the spline lock ring.
Remove the spline lock ring from the cassette. Take the chain whip off of the cassette. Pull the cassette outward to remove it from the spline on the hub body.