Lots of good information about adjustments etc in our FAQ:
http://www.hobiecat.com/community/viewforum.php?f=21
The basics are...
Mark the chain at the center sprocket tooth.
Loosen the chain(s).
Mark the drive axel on one end to help realign when replacing (runs length of the drive).
Loosen the set screw in the center of the drive axel.
Tap the axel aft or forward to allow the needed sprocket to be removed.
Install new sprocket centering middle tooth on the chain once again.
Tap axel back and spin to align set screw hole.
Tighten set screw.
Adjust chains.