How do you make a mercury 15 hp 2 stroke a 20 hp?

Step 1: find a pawn shop or boat dealer that has a 20-horse motor in good shape in stock.

Step 2: remove 15-horse motor from the transom of your boat, take it to said store and trade it in.

Step 3: go home, attach 20-horse motor to your boat, and be happy.

If the question was, "how do you make a Mercury 300 hp motor a 350 hp?" then the answer would be significantly different: there are a lot of things you can do to get 50 extra horses out of an engine (starting with replacing the stock prop with a racing prop). But only five extra horses? And on a motor this small? You're better off replacing the engine with one the size you want and using the time you'd spend working on the motor on tying new flies for next season.