First, check for sound to confirm the horn's electrical system works. If there is no sound, check the connections and make sure these are free from rust and other corrosion. If the fuse is burnt, replace it. If the horn still doesn't work, jump power to the horn by connecting it directly to the battery terminals. If the horn still doesn't make a sound, the damage is likely within the horn assembly and requires the assembly to be dismantled.
A horn assembly that comes with rivets instead of screws is difficult to dismantle and may need to be replaced. If the horn comes with screws, loosen and remove the cover, and then look for any broken or disconnected wires. Solder the disconnections back into place, and check if the horn works. Cleaning the contact breaker may also help.
If the problem is less-than-adequate honking, locate the horn's tone adjuster and turn it with a screwdriver until you get the desired sound. The tone adjuster is often located front and center of a horn assembly. Adjusting the contact breaker is another possible solution. The contact breaker adjuster screw is located to one side on the front or back of the horn assembly. Turn the adjuster screw until you get the sound you want.
Car horn repair can be noisy, so perform the troubleshooting at a location where you are relatively unlikely to disturb others.