Acesse o terminal SSH e digite os comandos abaixo:

Para instalar no CentOS:

wget http://prdownloads.sourceforge.net/webadmin/webmin-1.770-1.noarch.rpm
rpm -U webmin-1.770-1.noarch.rpm

Para instalar no Ubuntu/Debian:

apt-get install perl libnet-ssleay-perl openssl libauthen-pam-perl libpam-runtime libio-pty-perl apt-show-versions python
wget http://prdownloads.sourceforge.net/webadmin/webmin_1.770_all.deb
dpkg --install webmin_1.770_all.deb

Instalando com arquivo tar.gz:

cd /tmp
wget http://prdownloads.sourceforge.net/webadmin/webmin-1.770.tar.gz
gunzip webmin-1.770.tar.gz
tar xf webmin-1.770.tar
cd webmin-1.770
./setup.sh /usr/local/webmin