Per the Aux Manual (Comdtinst M16790.1G, p 3-54), submit a letter of resignation to the FC.
H.5.a.(1) Upon Request
An Auxiliarist may request disenrollment at any time at their prerogative. Such request must be in writing and submitted to the FC. The FC shall promptly forward all such requests to the Director, with a copy to the DCDR and the DSO-HR. Appropriate efforts should be made by the Auxiliarist’s chain of leadership to determine the reason for such request before it is forwarded to and processed to completion by the Director, but nothing shall unduly hinder an Auxiliarist’s desire to disenroll.
If an Auxiliarist still wishes to resign after consultation with the chain of leadership, then the Director will request return of the Auxiliarist’s ID card and the return of any Government or Auxiliary owned property in the Auxiliarist’s possession. Upon receipt of the written request, the Director shall remove the Auxiliarist’s name from the regional AUXDATA rolls and provide written acknowledgement to the individual of their request, service, and disenrollment.