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.
groupmod -n <newgroup> <oldgroup>
Look the “root” account.
passwd -l root