#usermod -l <newlogin> <oldlogin>
Ex: # usermod -l james john
Where john is the old login and james is the new login id.
$ man usermod
USERMOD(8) System Management Commands USERMOD(8)
usermod - modify a user account
usermod [options] LOGIN
The usermod command modifies the system account files to reflect the
changes that are specified on the command line.
The options which apply to the usermod command are:
-l, --login NEW_LOGIN
The name of the user will be changed from LOGIN to NEW_LOGIN.
Nothing else is changed. In particular, the user's home directory
name should probably be changed manually to reflect the new login
There is more to "$ man usermod" I just posted a bit of relivent info.
Hope that gives you an idea