I've tried following the instructions for an European 2014 model (Tekna) and went underneath.
However, I stumbled into the new honeycomb element and couldn't find an easy way to remove it. It seems to be very well attached to the bumper and front cross member.
That's when I tried to go from above and, to my surprise, it was much easier!
The cooling reservoir (?) is no longer in the original position. It is now on the other side of the car. Here's a picture (sorry for the low quality and light!):
After removing the plastic covers (hold by lots of plastic bits), you can find another plastic cover which can easily be removed after removing two plastic nails at the front and pressing the two buttons at the back (marked in red in the next picture):
Then, you have clear access to the horn:
The picture shows the new horn installed.
Mine is still not working because I'm figuring out the wiring (I'm waiting for a multimeter so I don't blow any fuse). Apparently, you need to use both wires from the OEM horn and I was trying to use the ground from the chassis and horn bracket (without success).
There two other curious facts:
1 - There are actually two horns on this Leaf! Even after disconnecting this first horn, I could still honk it. There's another one on the other side of the car and the access is about the same. The one I replaced seems to be the "low" tone while the other one, on the right-hand side of the car, seems to be the "high" tone.
2 - The OEM horns are "tunable", using a philips screwdriver:
I'm not sure if we can simply tune down the standard horns to improve the sound, instead of replacing them altogether.
I hope this helps other owners with more recent models.