You can use the usermod command e.g.
Code:
#usermod -l newlogin oldlogin
You can then make the user the owner of the previous users data e.g.
Code:
#chown -R newlogin:somegroup /home/oldlogin
I personally would rename the old home directory to the same name as the users login name e.g.
Code:
#mv /home/oldlogin /home/newlogin
and then do
Code:
#chown -R newlogin:somegroup /home/newlogin