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 the desired password by running;
/usr/libexec/webmin/changepass.pl /etc/webmin admin password_you_need_to_change.
It will then update the password. If it runs and sees that the user is not called admin, it will then tell you the name of the admin account user and then you can run the command again with the correct user.