Remove the spring from the two orange posts.

Spring is simply wrapped around the posts.

Removed large flathead screw marked in red. This may require a smaller flathead than the screw size suggests, as the gap is quite narrow.

The aperture lever assembly (Green arrow) can no be lifted out of the barrel.

With the spring and lever removed, you can push on the brass spring post circled in blue to remove the final piece of the aperture assembly for cleaning.