You don't really need to take the bumper off but it helps.
I changed both my horns with Hella Supertones about a year ago. I took out all the lower grilles and opened the hood. You should be able to get your hand and a wrench in there to take the horns out. It's just a small bolt or nut, but I can't remember.
The Hellas are hard to mount because they can't use the stock location without a special bracket. I suggest getting some aftermarket horns that fit in the stock location. I also had to cut off the stock connector and use simple crimp connectors to connect up to the Hellas.