How do I upgrade to the latest version of webmin?
I installed virtualmin with the install script on this site. Does virtualmin package updates take care of any future webmin updates? or I need to do something to keep webmin up to date.
You can do both.
Virtualmin will track all updates for you. It will inform you about (and allow you to install) updates for Vmin itself, for Webmin and Usermin, and also for other system packages.
If you wish to, you can also still use the package manager of your Linux distro (like Aptitude for Ubuntu/Debian) to fetch updates.