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There is a slot. Insert a flat blade screwdriver which is to hold the ring in place while you rotate the visor toward the door side (like your blocking sun coming from the side). It will drop down a notch, then rotate it slightly more and it will drop out. Disconnect the electrical connector.
I'd have to go look but I seem to remember it says something about installing a new ring. The old will probably work. Just reverse the procedure to install.
I wrapped tape around the arm of my visor during removal because I was reinstalling the originals.
Hold the flat-bladed screwdriver in the ring tab (or slot) and as the other poster said, move the visor towards the door as if you were going to block out the sun. The arm will push on the screwdriver shaft, loosening the ring.