Below tutorial will show you how to change username in ubuntu 12.04 precise.First,we need login as root the change the username,then restart the enable this changing ,here we go.
1.Open a terminal by Pressing Ctl+Alt+t or search terminal in Dash
2.Unlock account root and enable login as root using below command
sudo passwd root sudo sh -c 'echo "greeter-show-manual-login=true" >> /etc/lightdm/lightdm.conf'
3.Reboot your computer and login as root
4.Open a terminal and Use blow command to change your username
usermod -l newname oldname
5.(Optional)Now you already changed your username,but your home folder name still are “/home/oldname”,if you also want to change /home/oldname to /home/newname just run below command
usermod -m -d /home/newname newname
Note:Of course you can combine step 4 and step 5 using below command
usermod -m -d /home/newname -l newname oldname
6.All done,use below command to lock your root again,then you can reboot and login as your new username
passwd -l root