First, edit your sources file to add the Webmin repository:
sudo nano /etc/apt/sources.list
Add the following two lines at the end of the file:
deb http://download.webmin.com/download/repository sarge contrib
deb http://webmin.mirror.somersettechsolutions.co.uk/repository sarge contrib
Next, add the necessary key to ensure smooth access:
cd ~
wget http://www.webmin.com/jcameron-key.asc
sudo apt-key add jcameron-key.asc
Now that we’re all set up, time to do the actual download and install:
sudo apt-get update
apt-get install webmin
apt-get -f install

Done. You should now be able to login to Webmin at the URL http://localhost:10000/. Or if accessing it remotely, replace localhost with your system’s IP address. You can login in with any user account that has sudo powers.