OK, if you look in your circle at about 5 O'clock, you'l lsee the small head of a pin in a small square recess.
This is where the fun starts - you need to remove that pin, which engages in a groove in the big pin that the piston end rotates round. Once you have the small one out, take the pressure off & the big pin will fall out.
I used mole grips & a gallon of WD40 on mine. Granted I had the advantage of doing it on the workbench with the rest of the engine scattered around various boxes, but as Hightower says, worth propping the engine to prevent headaches.
(There's a similar pin at the other end)