Turn the lower Piston-Gear a bit like shown on the picture. Now should fall off the two rings which I allready told earlier.
Then use the srewdriver to push out the lower Piston from the lower gear mechanic. Please note the POSITION of the Piston because of the orientation of the nozzle where the water flows into the brew-group. If it's mounted wrong at the reassembly the brew-group will break !!!