Allow me-- did this drill a while back. You can do it without removing the wheel.
Remove the right side cover and remove the rear master cylinder mount screws, but not the hydraulic hose. Just let the master cylinder droop out of the way.
Remove the battery--there is an access hole in the battery tray for the horn attachment bolt. Remove the bolt with a socket and extension bar. This allows the horn to fall on top of the swingarm, still connected to the wires.
Now that the horn is loose on top of the swingarm, use your fingers, needlenose pliers, whatever to fish the horn over to the right side. Use the opening you created by loose-removing the master cylinder. Hold on while you unplug the wires and remove the flat bar horn bracket from the horn body. It won't fit through the opening with the bracket still attached. You have to orient the horn just right but it can be squeezed out. Be patient--it really will fit if you turn it just so. Leave the flat bar attachment bracket in there, or remember to re-insert it first before the new horn. You'll have to re-assemble the bracket onto the new horn when the horn is in the space under the battery.
Make a note of this orientation and use the same one when inserting the new horn in there. It will require scraping it against the plastic fender extension to the point of flexing that plastic some, but it can be done. Then fish it over to re-connect the wires, re-attach the horn bracket, and re-attach the mounting bolt through the battery box. You'll need something like a magnetic probe to start the nut. Think surgery.
Tip--when you re-attach the horn, use a plastic/nylon or rubber washer between the horn bracket and the mount under the battery box. Without a cushioning washer, if it gets too tight, it won't work, and there's not much leeway between "so loose that it won't stay attached' and 'too tight to allow proper vibration so the horn still works'. I used a nylon washer, tightened the bolt and the horn works fine all the time.
Hope this helps.