The horn cap is really very thin.. If you are trying to remove everything above the arms.. that's not how you do it.. But first an aside
By careful running of the seat forward and back as you turn it you SHOULD be able to rotate it without removing the wheel,,, Just put the wheel in the "Full Up' Tilt position.
now, to remove it. As I said the horn cap is very thin, The edge, as I recall is not much thicker than a sheet of typing paper.. It is just the TOP of the CAP..
After that.. Standard job, just like most any other vehicle.
Home is where I park it!