After the installation of Webmin was finished, you may want to enter to the Webmin tool front page with the default user name (root in many cases) and your password. If you don’t know the password or Webmin not allows you to enter to your account, you can change the password with command prompt.
This command should work on Linux Ubuntu or Linux Debian.
# set / change the default pass of webmin using the command line.
# change the "your_password" with your password:
/usr/share/webmin/changepass.pl /etc/webmin root "your_password"