1) Log out from current session and log as root user.
2) If you cannot log as root user add another account temporary and log in with it.
3) Run "sudo usermod -l <newuser> <olduser>" from that account.
4) Make sure new account can run sudo. Run this to add new user with sudo privileges. "sudo adduser <username> sudo". This need in case of you cannot log as root.
If you need to change home directory too run this "sudo usermod -d /home/newHomeDir -m newUsername"