With an H5 hex drive, remove the four large bolts holding the coffee press to the bottom of the heater. It is seen on the left of the photo)

Remove the water sieve by undoing the cross-head screw. This exposes the brass exit valve from the heater.

Remove this brass exit valve, on top of it is a spring with a bullet shaped seal. In my case it revealed a LOT of calcium build up.

Finally split the top from the bottom of the heater. In my case this was easy, you may have to use a blunt tool to help lever this. Remember it is die-cast metal = brittle, so dont use excessive force. This exposed the crud in my heater.

Remove the damaged "o" ring