WARNING: This page is about a different car, the 2003 BMW X5, 2002 BMW X5, 2001 BMW X5, and 2000 BMW X5. However, it is still accessible from the selected car via links, so may be relevant.
Unscrew nut (1).
Fig 1: Removing Horn
Courtesy of BMW OF NORTH AMERICA, INC.
Remove horn (2) from fixture and disconnect plug connection (3).