You can update your file until Jan. 31 via email or snail mail. If it is a school activity, you need to have your school counselor endorse your submission (just as you had to do with your Candidate Activities Record) or he/she can send it directly. If it's something with objective proof (i.e., a certificate from a well-known organization with your name on it), you may be able to send that by itself but USNA may still want endorsement. You can confirm with Admissions.