How do make a sudden rash (hives/urticaria) go down as soon as possible? Urgent.?

You'd be a fool to do a spray tan with hives ( or any skin problems). I'd concentrate more on trying to figure out what caused the problem ( and I can guarantee, skin tanning isn't helping your skin one bit, your willing to trade your health for temporary beauty ? That's a young mind at work for ya. Prom will be dark, no worries.