If you insert a screw deliver with a wide (about 5mm) blade next to the head itself, between it and the plastic "grommet", it magically pops off. Just be careful.
If you plan to replace, pop it back on and off again to give you confidence that it is an easy thing to do.
Michael Kenward
December 2010
Bring you fingers close to the snap joint and pry off using your fingers. If your fingers are too large, use a butter knife with a cloth around it to protect the finish of the arms. Slide it toward the joint between the connecting arms and twist outward. Remember: The arms are simply snapped on. An Artemide representative told me its like a strong snap from a pair of blue jeans.