XN250 O-Ring replacement

Greetings,

My Nespresso U Krups XN250 suffers from water leak. Looking at how the machine behaves with the cover removed I do suspect O-Ring is torn. I'd like to try to replace it, but I just can't reach the ring. It looks like in order to do it I need to remove moving part (piston? MS-623324, see the picture), but I have absolutely no clue how. I dont see any screws, cant move gears manually to move the part to the edge position etc. Please advise.