Replacing rear sprockets begins by identifying the system. A cassette slides onto a freehub body and is retained by a lockring; a threaded freewheel is removed differently. Before reaching for a wrench, establish which system is on the wheel and which lockring profile the tool must fit.
On a conventional cassette, a chain whip holds the sprockets while a matching tool turns the lockring. Once the ring is loose, lift the cogs and spacers off carefully, keeping their order. That small step matters: a misplaced spacer can alter the arrangement of the entire cassette.
When refitting the cogs, the spline pattern on the freehub and sprockets guides their orientation; the wider spline shows how they align. Start the lockring gently and tighten it to the relevant manufacturer’s specification. If the cassette or wheel has changed, check the rear derailleur’s operation after assembly.