From the FAQ: http://www.webmin.com/faq.html
Quote:
How do I change my Webmin password if I can't login? Included with the Webmin distribution is a program called changepass.pl to solve precisely this problem. Assuming you have installed Webmin in /usr/local/webmin-1.150, you could change the password of the admin user to foo by running /usr/local/webmin-1.150/changepass.pl /etc/webmin admin foo
