sudo apt install apt-show-versions libapt-pkg-perl libauthen-pam-perl libio-pty-perl libnet-ssleay-perl
Get the latest version here: http://webmin.com/download.html.
wget https://prdownloads.sourceforge.net/webadmin/webmin_1.881_all.deb
sudo dpkg -i webmin_1.881_all.deb
Webmin is now installed.
You can access it on https://yourserver:10000.