A newer style freehub body is the ratcheting system that holds the cassette to the wheel hub. It is an improvement over the older style freewheel hub in that you can replace gear sprockets without having to change the whole mechanism. The freehub body should last a long, time as most of the components will be weather sealed, but if the freehub does wear out, it can be easily replaced. This guide assumes you have a quick release rear wheel, but the procedure is the same if your wheel is attached with wheel nuts.
This is a long guide, so you can choose to either watch the entire video above, or follow it in smaller chunks by clicking on the individual steps below.
Steps to Replace A Freehub Body:
A freewheel is a ratchet mechanism mounted on the rear hub which allows the chain to drive the wheel forwards via a cog but when the wheels move faster than the chain the cog is not driven round by the wheel. A freewheel allows a rider to rest their legs while the bike is moving forward.