Keep it warm, bring inside, and try to get the chicken to drink plenty of water. Make sure your coop has good overhead ventilation, and prevent ammonia odors from droppings, and keep bedding dry and clean. Respiratory diseases in chickens can last weeks. Are there any other symptoms besides coughing?
Can you take her to the vet for antibiotics? Are you an adult or teen? Some diseases are caused by viruses, and antibiotics will not help. Some are bacterial, such as MG or coryza, and those may respond to antibiotics. You can get Vet Rx at feed stores, but it really has nothing in it to treat a respiratory infection.
Can you tell me if she has any bubbles in her eye or eye drainage, or runny nostrils? Any pictures or a video that you can post to YouTube? Chickens can have 1 low dose 81 mg or baby aspirin for pain once or twice a day. If you have 325 mg aspirin, cut into 4 pieces and give a piece in some cooked egg or food.