ah ya vi como , ahi les va:
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-1.290, you could change the password of the admin user to foo by running
/usr/local/webmin-1.290/changepass.pl /etc/webmin admin foo
fuente :
http://www.webmin.com/faq.html