As you known, It is easy to change the root password in Ubuntu. This post tells you how to change computer name quickly.
For example, I want to change my computer name to a new name i.e. vntutor

  • System -> Administration -> Network
  • Select the General tab
  • Enter new name vntutor in the Hostname field
  • Click Change Hostname in pop up dialog
  • Click Close in Network Setting window
After this setting is done, file /etc/hostname will be update to vntutor. Another solution is that I use this command: sudo hostname vntutor but it doesn't work because file /etc/hostname still keeps old value. :)