The only thing I can think of to do (without disassembly) is use an old toothbrush to brush the upper part of the aperture ring. There's a space there, maybe 1mm, which might have some grit jamming the ring.
Other than that, find a decent screwdriver, preferably JIS, and unscrew the mount screws. Then you should see this:
There's a spacer/washer that I always set aside and forget later. The photo helps figure out how to put it back on. You can slide off the aperture ring now but be careful of the 2mm bearing trapped under the orange diamond:
The problem should be obvious here. The aperture ring is metal so if you don't see any grit, check Steve's theory. It might not be round any more.