Old spinner fell after washing the dishes.

Carefully unscrewed the top screw holding the spray arm so it doesn't fall into the tub.

Pull down the arm gently and pop out any remaining parts from the old spinner.

While the spray arm is pulled down, snap the two parts of the spinner together: one from above the spray arm, and one from below the spray arm. (you need to snap them together before you screw the spray arm back, because it won't snap together otherwise.)

Rescrew the spray arm and you are done.