This is for if its the actual shoulder joint, if its for the bit under the shoulder bell I'm not sure how to get the individual arms apart, knowing medicom they're designed never to come apart without breaking (interestingly, sideshow's stuff always comes apart very easily!)
Take screws out fully and gently pull apart. Not sure if some are glued, or even if mine was, but if it was then it gave up easily the first time!
Be very gentle here as the wires for the lights are soldered on so minutely, a minor tug could break it
It actually looks like a droid inside
Then be careful that the wires don't get trapped as you recombine the torso!
It isn't as fraught as I make it sound, just adding lots of warnings as given how some people handle these things, you'd think they were dealing with a Stretch Armstrong
But have a play around to see what makes it tighter, try a thin layer of gloss varnish, nail polish or even paint. Not too much, and leave it to dry before reassembly!