How to replace a ICI Caldaie Solar 96E diverter valve diaphragm?

I have a slow leak from one of the screws heads at the bottom of the diverter valve. I have ordered a new diaphragm but am not clear how the top half of this unit (ie flow switch side) is disconnected from the heat exchanger plate. The service manual does not describe how to replace the diaphragm, any help would be appreciated.