As always, the old adage holds true. If you leave them alone, they'll leave you alone. And of course, a general rule is not a promise. Sometimes bees get in a nasty mood and you never know for sure what set them off. In such cases it might be that the trigger happened before you arrived.
Most swarms move on within a day or two after they arrive.
If you find one that is in a fairly secluded or high location, you don't really have to do anything about it. There are a couple of potential drawbacks to the do-nothing approach, though. If the bees don't find a suitable home, they may never leave. Very occassionaly working colonies have been found hanging from exposed branches of trees. If this happens, the bees will become more defensive since they once again have a home to protect.