At some point during the ownership of your Roadster you may need to replace the rear cog. Belts, chainrings, and cogs are all consumable items that will wear out over time. With these few steps you will know everything needed to replace the rear cog and keep your Roadster running for years to come. 

There is a locking ring that goes on the end of the wheel. This tightens down to hold the cog on the wheel. All you have to do is loosen that ring and the cog will come off. The only tricky part is that the cog turns when you try to turn this ring. In order to loosen the ring you have to hold the cog with a strap wrench and then turn the lock ring. There are two tools that are needed to do this, a cassette tool and a strap wrench 

The following picture and video link go over how this process is done.