Go to bassboatcentral.com, they have great step by step instructions with pictures to replace an impellor. As long as you have some mechanical inclinations you should have no problem. I'm no mechanic and I replaced the impellor in my 150 Merc EFI with only the instructions from bassboatcentral and the manual. It really was a piece of cake, I wouldn't think the 200 would be that much different.
The only tricky part was getting the shift shaft and drive shaft splines to line up. The secret to that was keeping in the shifter in the forward position when you remove the cone and gently moving the shifter back and forth to get the shaft to line up when putting it back together. The nosecone should just slide right on and once the shaft is lined up it should easily push together. Do not force it, if it doesn't slide together without force then its not lined up right. It may take a couple of tries to get it right.
A couple of suggestions:
1.) Make sure you have someone to help you and take the prop off before you start this will reduce the weight of the nosecone.
2.) If you don't have a Merc manual see if you can borrow one and get copies of the instructions from the manual. They are also really helpful and the manual for my motor actually had a couple tips for making it go back together easier.
3.) Make sure you have a torque wrench, the impellor housing should be torqued only as tight as the specs suggest.
Good luck.