The screws holding these pieces together are in there very tightly. Make sure your screwdriver has sufficient 'bite' or you will strip the head (and then you're in deep trouble). You may also have to push down quite a bit to get the screws to turn. Find a friend to hold the lens steady while you do this.

Unscrew the five screws around the top of the light gray ring. This will free the lens unit from the focus barrel.

Put a tab of masking tape, or an elastic band, on the lens hood (not the focus ring). Mark the location of the ribbon cable. This will save you much grief during reassembly.