Slide over the propeller and prop washer, and screw over the castellated nut, lining up a slot inside the nut with the outlet for the cotter pin. Insert a different cotter pin and bend the ends.
Unscrew the duvet plate https://aprilczuo085011.tblogz.com/yamaha-outboard-motor-for-sale-can-be-fun-for-anyone-44341330