Thats easy. First check if the fishes insurance covers it.
Put the fish in a net just on top of the water. Take a small hypodermic needle and make sure the plunger is all the way in. Stick the needle just in the stretched membrane near the top of the eye. Don't go in to far although there are no organs in there but the optic nerve is in the middle of the eye. The bone will prevent you from going into the brain but you want to just pierce the membrane. Pull back on the plunger and the eye will go right back where it is supposed to be. If it doesn't go all the way in, do it again the next day, don't keep poking the fish.
If air comes out, don't worry about it.
If fluid comes out also don't worry but that means an infection and you may need to get a little injectable antibiotic. If you can't get it, don't worry, it will probably still go away.
If you can get injectible antibiotic. After you get the fluid out. inject a little antibiotic in the same spot, just enough to protrude the eye. then suck it out. The fish will thank you.
People tell me thats cruel. If your eye was hanging out so that you could see forward and backwards at the same time and your friends run away yelling MONSTER and I said I could cure you in 3 seconds with no pain, what would you do?
I doubt you would say " Oh No leave my eye like that, I could get on the cover of Weird Eye Weekly Magazine or star in a Monster flick"
There's pictures of it in my book, but you don't need it because I just explained it just the way it is so make believe there is a picture of a fish here.