First, you need to determine what is causing the rash. Many rashes disappear on their own in a few weeks, but there are things you can do to reduce symptoms. Stop using your deodorant for a couple of days to see if the rash goes away, being sure to gently clean the area regularly. Keep your underarms dry and wear light, loose-fitting clothing. Avoid going outside in hot weather or at least keeping exertion to a minimum outdoors. Dab on calamine lotion, apply a hydrocortisone cream, or take an antihistamine.