If it happens again, try a brackish bath to draw fluids out of the fish as temporary relief. Guppies love brackish water anyway, so if that's all you have in the tank, might as well just run it brack.
As for curing a symptom like dropsy, you'd have to get a positive ID on the ailment. And good luck with that one unless you have a microscope and experience in aquatic pathology. Some aquarists have had luck with sudden high dosing of powerful antibiotics like nitrofurazone and the like. Sounds like a waste of time to me, though. By the time you see dropsy, organs have begun to shut down.
I just euthanize dropsical fish these days. Thankfully, that only means bettas. Nothing else I have gets dropsy, which implies a latent pathogen that targets bettas. But, again, without a degree in microbiology, I've got nothing.