Yep, like EK Mad said, a very small thin screwdriver will prise the horn button off.Be carefull though as the edge of the button can be brittle so prise the screwdriver in gently and do it from the bottom area of the button so if it does chip the edge a little it won't be as noticeable at the bottom.
And Reidy's suggestion of a steering wheel puller is the right way to get it off.Use the puller if you can because it will help stop the wheel from cracking by using the alternative method I'm about to describe below.
If you can't get a puller then try and buy/obtain a copper headed hammer/mallet or a brass drift ( piece of solid brass rod ) about 6 inches long.Unscrew the nut holding the wheel on to the shaft but don't unscrew it completely, leave it still on the thread so it is level with the end of the shaft.
Get someone to pull on the steering wheel whilst the other person hits the end of the shaft/nut ( not too hard if possible ) till the wheel releases off the shaft spline.Then just undo the nut from the shaft and pull the wheel off.
The reason you leave the nut on is so when the person is pulling on the wheel it doesn't let go all of a sudden and hit you in the nose and break your nose like I did when I was an apprentice

.It will still let go all of a sudden most times but the nut keeps it from modifying the honker.
Copper headed hammer/mallet or brass/copper drift is used because these metals are softer than steel and when you hit the end of the steering shaft with them then the thread on the shaft doesn't get damaged ( most times ).
Sounds difficult but it's not really.It's only difficult to describe in words how to do it without using pictures.
Hope that helps, and cover your nose

,
Terry.
P.S. Before you take the wheel off also mark with say, a black texta, the steering wheel and the end of the shaft so that when you replace the wheel then it goes back on in the same position on the spline as it was before.Otherwise if the wheel goes back on in the wrong position the steering wheel will be positioned wrong when you are driveing straight ahead.And that is a pain in the bum when that happens!!
I can't think what to write here so this will do.