To upgrade Webmin, you need a webmin installation on the Linux first. 🙂
Say, from your desktop browser, enter https://yourlinuxbox:10000
It is security connection.
Then, do if as below:
To have updates installed automatically, go to the Webmin Configuration module, click on Upgrade Webmin and use the form in the Update Modules section.
The latest version of Webmin is 1.430