How do you treat a 'fever' that is a result of being in heat for too long?

Hard to answer: Body temperature raised as result of exposure to the heat can run the whole range of treatments: if it is a small fever, say under 102, without other symptoms: get into shade, cool area, hydrate a lot. If other symptoms/higher temp involved, hydration is imperative, cool the neck, armpits, groin area and possibly seek med attention. If heat stroke, it is a 911 emergency, the younger the worse!

Cool off: If that is truely the only reason for increased temperature, then cool shower / swim / tub are best option. Be careful, as heat exposure enough to raise temp can cause heat injury / exhaustion or stroke.