SSH onto your server
Get the latest version (1.510 to date)
wget http://prdownloads.sourceforge.net/webadmin/webmin_1.510_all.deb
Extract and install the package
dpkg –install webmin_1.510_all.deb
It may error on a few dependencies so to overcome those install these
apt-get install perl libnet-ssleay-perl openssl libauthen-pam-perl libpam-runtime libio-pty-perl libmd5-perl