How can I treat altitude illness?

Safe descent. 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.
Lower altitude. High altitude illness is a serious issue and I am not sure I understand what you mean by altitude illness. The treatment for high altitude sickness is evacuation to a lower altitude, diuretic and oxygen.