how to change the VM root password from command line?


I’m running VM pro on my main server and VM gpl as secondary. Normally I access the secondary using the “Webmin Servers Index”. Because of an error, I changed the password for the VM gpl root user. The password of the unix root is still valid. At least I can login using it via a console. Assuming that the VM password is the webmin password, I tried to set it according the webmin documentention ( /usr/libexec/webmin/ /etc/webmin root ). The command seemed to work, but I still cannot login to VM using the new password.
I assume that there is a way to reset the password, but I didn’t find it. Can anyone help me with this issue?