Start with the easy option
a multimeter set to 12v DC (most are 20v DC which is fine)
Put the probes on the horn terminals and press the horn
should get atleast 12v (maybe less with engine off)
If you get 12v - its the horn
if you dont get 12v its something else
The Fuse is a 15amp fuse
Engine bay box - Right hand side of the Mini Fuses (Small ones) its the 4th one down
If that fuse is okay
Check the relay - its the the little boxes Near the fuse actually
Its the 3rd one down (black) On the right hand side
I believe the Hi-beam relay will be fine for testing - so swap it with the other black relay (top one) to ensure the relay works fine
If the relay is at fault - the Hibeam lights will fail to work
but relay failure is rare to be honest, i'd be inclined to test the horn with a direct 12v feed before playing with relays
Ziggy