This is a DIY on how to replace both high and low tone horns.
I personally used these horns, and i recommend them to anyone looking to do this, as they are deeper in tone and loud.
http://www.amazon.com/gp/product/B000DINKPQ/ref=oh_details_o06_s00_i00?ie=UTF8&psc=1
http://www.amazon.com/gp/product/B000FKKOK6/ref=oh_details_o06_s00_i01?ie=UTF8&psc=1
I'm not sure if these instructions will match with every kind of horn available, but most are universal, so i think it should

I did this on my Si sedan, and I think it's the same across all trims, except the LX(or EX?) only having the high tone.
Right then, onto the DIY.
Step 1
Remove your bumper.
I wont go into this because College Hills Honda has a great video to show how to install fog lights, but the first part of the video shows you how to take off the bumper.
Step 2
Locate your high tone (right above the radiator)
First, unplug the connection to the horn (#1)
Second, unbolt the bracket in #2
There isn't a lot of room for a much bigger horn (without some modification) I personally had to bend that bracket (#3) to get my new horn to fit.
Step 3
Follow the instructions again and u
Now go and beep at random people with your new horn!