How do you convert a 2002 Honda 9.9hp four-stroke outboard to 15hp?

Hello hwk559, I was interested in your question because I once had
instructions for converting my 85 HP Evinrude to 115 HP. I found your
answer in a Honda outboard forum and it is a very simple task.
 
http://www.geocities.com/blew_skies/honda1.html

Yes, Virginia, the 9.9 is a 15 in sheep's clothing. The only
difference between the newer Honda (97) 9.9 and the 15 is the throttle
stop screw and the two jets in the carb. The newer jets are 52s with
the old being 48s. I have done this on my 9.9 and it is much happier
now. Your local Honda dealer should be able to order the jets for you.
The two jets are @$20 for both and well worth it.

You need to read the entire article. Apparently you can get 13.2 HP by
just removing the throttle stop screw. The whole purpose in Honda
derating the engine is to comply with the 10 HP limits required on
some lakes.

Redhoss