Change Just the User Name
To change a user’s login name you can use the following command:
usermod -l new_username old_username
Change User Name and Home Directory
To also conveniently rename their home directory:
usermod -l new_username -m -d /home/new_username old_username
Change the Group Associated With User
To change the group name that gets associated with the user (you’d probably want to do this whether or not you change the home directory) use the following:
groupmod -n new_username old_username
References
StackOverflow - How can I rename an unix user?