The "rivet" is actually attached to the arm on the cross shaft. The "rivet head" just keeps the rubber donut from falling off the arm. Soak it with some rubber lubricant and pry it off. Pry the donut out of the eye. Lube the new one and work it in. Lube the "rivet" and slide (more like force) the donut onto the rivet. I like to use dish detergent and water as a lube. Others like silicone.
Vince Canepa
1967 250SL
113.043-10-001543
568H Signal Red
116 Caviar MB-Tex