The easy way to install Virtualmin is to run the Virtualmin install script on a freshly installed supported OS.
The reason there isn’t an upgrade button is because a Webmin installation is simple. It is a web-based UI for whatever it finds on the system. Virtualmin is expected to provide the whole dang stack of applications and the UI and they’re all supposed to work together in a friendly way. Completely different goals.
You can install the Virtualmin virtual-server module into your existing Webmin very, very, easily. If you installed Webmin from a package (rpm and deb), you should also download the same type of virtual-server module package from the Virtualmin repo (Index of /vm/6/gpl) and install it. If you used yum or apt, even better…use the install.sh with the
-s flag to setup the repo, and then install the package.
But, this won’t setup anything for you. It won’t be a “Virtualmin system”. It’ll just be Webmin with the main Virtualmin module installed, and you’ll need to configure it appropriately for however your system is setup to host sites (which is probably different than what our installer does…there are literally infinite possibilities). As long as the Apache (and some other) modules are configured correctly in Webmin, it might Just Work, mostly.
I’m not recommending you do this…but, it’s the easiest and safest way to get Virtualmin (by some definition of Virtualmin) on your already production system. Other modules can maybe be installed, as well, but with the same caveat that you’ll probably have to do some configuration yourself.
It may be easier and better for you in the long run to install Virtualmin the usual way, using the automated install script on a freshly installed supported OS, and then migrate the sites and data over from the old system. That’s what I’ve done in the past when I needed to help a client go from an ad hoc self-managed system to a Virtualmin system.