Now write "TOP" on the white gearwheel. When it's mounted upside down when reassembled it can't rotate properly. Really truth.
Turn off the Gearwheel and pull out the upper brew-cylinder (Piston).
Now you can change the seal-ring.
If a broken seal was the problem and you don't wish to dissect the brew-group to the end you can now start to reassemble the way backwards. :) But think about it: At the lower brew-group-piston is a sceond seal-ring.