Change the username
At the start screen press
Log in using your username and password.
Set a password for the "root" account.
sudo passwd root
Log in using the "root" account and the password you have previously set.
Change the username and the home folder to the new name that you want.
usermod -l <newname> -d /home/<newname> -m <oldname>
Change the group name to the new name that you want. I'm not so sure about which group should I change to
groupmod -n <newgroup> <oldgroup>
Lock the "root" account.
passwd -l root
If you were using ecryptfs (encrypted home directory). Mount your encrypted directory using
ecryptfs-recover-private and edit
<mountpoint>/.ecryptfs/Private.mnt to reflect your new home directory.
Change the hostname, which is the computer name
Type the following command to edit /etc/hostname using nano or vi text editor:
sudo nano /etc/hostname
Delete the old name and setup new name.
Next Edit the /etc/hosts file:
sudo nano /etc/hosts
Replace any occurrence of the existing computer name with your new one.
Reboot the system to changes take effect:
Change the password
Super easy, isn't it?