Log into your server
wget http://downloads.sourceforge.net/project/webadmin/webmin/1.510/webmin_1.510-2_all.deb
sudo dpkg -i webmin_1.510-2_all.deb
If you get errors, just type sudo apt-get install -f (this will install all the required pre-reqs)

And then I usually update the server:
apt-get update
apt-get upgrade
apt-get dist-upgrade

And then make sure you change the security on webmin:
Login to https://yourip:10000 as your root user
Click on Webmin, then Webmin Configuration
Click on IP access control
Only allow from the following:

127.0.0.1
192.168.1.0
OtherIP
ETC

***EDIT***
http://downloads.sourceforge.net/project/webadmin/webmin/1.570/webmin_1.570_all.deb
http://superb-dca2.dl.sourceforge.net/project/webadmin/webmin/1.580/webmin_1.580_all.deb