/etc/passwd. This may have sufficed in the old days, but not any more. For starters, much of the info here is mirrored in
/etc/master.passwd. Further, the data is also now maintained in a database, which can be updated by 'pwd_mkdb'.
However, you don't really need to know all this, simply use
vipw any time you wish to change user info. (Use
'man vipw' for more info.)
vipwto rename the account.
mvto rename the users directory in
mvto rename the users mailfile in