The N-400 form should be sent to either USCIS, P.O. Box 21251, Phoenix, AZ 85036, or USCIS, P.O. Box 660060, Dallas, TX 75266, states the USCIS website. This form is to apply for United States citizenship, and the address to use depends upon a person's state of residence.
The website for the United States Citizenship and Immigration Services contains a list instructing which of the addresses to use for each state. All persons associated with the military, and their relatives, must send their forms to Nebraska Service Center, P.O. Box 87426, Lincoln, NE 68501-7426. The USCIS website also gives addresses for applicants who want to submit their applications via courier or express mail services.