To remove the lens core from the innermost black barrel, push one of three pegs towards a slot in the flange. With light pressure on the front of the lens (but not on the protective lens shutter), all three pegs will pop through their slots, allowing the lens core to be removed.
To reassemble, ensure the lens is fully closed. Hold the silver barrel in the fingers of one hand and rotate the rearmost component anticlockwise against spring pressure with the fingers of the other hand. Extend the grip of the fingers of that hand to hold the lens against the spring.
Ensuring the slots in the innermost barrel through which the ribbons have to pass are roughly aligned with the ribbons, and insert the silver barrel into the innermost black barrel. Once the pegs are in the slots they came out of, allow the spring pressure to feed the pegs into their channels.