Sounds like the umpire called the Infield Fly too soon. You should really wait until the ball has reached it's peak and is starting back down, so that you can be sure of where it might land and if an infielder will have a routine play on it, before you even think about announcing the Infield Fly.
But apparently this guy didn't. So what do you do now?
Rule 10 (which covers the umpires) says that the plate umpire may rectify any situation where runners or fielders are put into jeopardy by a reversed or delayed call. The "fix" can be any combination of enforcing outs made on the play, or eliminating outs, or placing runners on an advanced base, or even returning them to the bases they started on.
In short, there's no "set in stone, one size fits all" solution here. The umpire needs to judge what the most likely outcome of the play would have been had he not reversed the call and confused the players, then place runners or enforce outs accordingly.
Doing that really requires seeing all of the play and how the players reacted to the first (wrong) call. You say this ball went into the outfield and dropped in front of the right fielder. Was it a ball where the runners would have been off and running had the Infield Fly not been called? Was it a ball where you would have expected the runners to have been staying put anyway, because there was a chance of it being caught? Was the ball hit deep enough that the runners probably would have moved up, or was it so shallow that they couldn't have advanced? If the runners would have normally been tagging, do you think that the right fielder would have had a shot at getting a force out, say at second base? How did the other runners react to this call- did they all stay put or take off? How about the defense- did they "stop playing" when the Infield Fly call was made, which could have also put them at a disadvantage?
Okay...you're the umpire. Take all of that into consideration, then place the runners or enforce any outs necessary to duplicate whatever you judge would have most likely happened if the right call had been made in the first place. Good luck, because no matter what you choose, you can almost guarantee that one side or the other is not going to like it! You're probably going to get an earful and, to some extent, you're just going to have to take it because this whole mess is your own fault!
Believe me, it's so much easier to just call this the right way in the first place. All this umpire had to do was say nothing until he was totally sure where the ball would be coming down. Do that one simple thing and this whole mess is avoided. I hope he learned a lesson!