Drop in elevation. The best treatment for altitude illness or altitude sickness is to decrease one's elevation by at least 1000 ft (quickly and safely.) if symptoms do not improve, one needs to descend another 1000 ft. If one has symptoms of hace (high altitude cerebral edema) or hape (high altitude pulmonary edema) contact search and rescue and arrange for emergent air evacuation.
Caution. Zone of tolerance: go too high above what you are prepared for & you get sick. This "zone of tolerance" moves up as you acclimatize. Each day, as you ascend, you are acclimatizing to higher elevation, & thus your zone of tolerance extends that much higher up mountain. The trick is to limit your daily upward travel to stay within that tolerance zone & to use medicines to help push beyond-too risky!