Theres a threaded lockring on the driver that locks the cog and spacer down onto it. The cog isnt threaded so dont use a chainwhip. you need a spanner (i think its 17mm) on the lockring, loosen it off any way you like, take cog off, put new one on, tighten lockring.
Also watch out the lockring doesnt loosen off on any rides afterwards - mine did and i needed to get a new one, but ive now got loctite on it.
Noz.