How to change username in Ubuntu

In order to change your old username (old_user) to a new one (new_user) do the following:
(I tried this logged in as a different user)

$ usermod -l new_user old_user

Now change the name of the home directories:

$ mv /home/old_user /home/new_user

Next, edit the /etc/passwd file, look for new_user and change the home directory to the new one.

Now, reboot your computer.