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 erecisely this problem. Assuming you have installed Webmin in /usr/libexec/webmin, you could change the password of the admin user to foo by running

/usr/libexec/webmin/changepass.pl /etc/webmin admin foo

 

sudo /usr/libexec/webmin/changepass.pl /etc/webmin root yourpassword