Do you see the little (5mm) hole in the top of the round disk? The corresponding one on the other side? This is a retaining nut, and that's how you loosen it. From the ARCO page on maintaining your winch
Insert a 3/16" screw or a small screwdriver or similar, into one of the two holes.
Insert your winch handle into the Main shaft of the winch and turn the handle against the screw, in a CCW direction, to unscrew the Retaining Nut.
Turn the winch handle in a CW direction to tighten the Retaining Nut.