If you like to install and update Webmin via APT on debian.
Edit the file on your system :
and add the line :
deb http://download.webmin.com/download/repository sarge contrib
You should also fetch and install GPG key with which the repository is signed, with the commands :
cd /root wget http://www.webmin.com/jcameron-key.asc apt-key add jcameron-key.asc
You will now be able to install with the commands :
apt-get update apt-get install apt-transport-https apt-get install webmin
All dependencies should be resolved automatically.
More info: http://www.webmin.com/deb.html