The page http://doxfer.webmin.com/Webmin/WebminConfiguration shows the webmin configuration screen with an upgrade icon similar to the usermin configuration screen. I see all those icons in my webmin but not the upgrade arrow. I don't see a missing image box either so it's not just a missing icon. The upgrade feature is missing completely. The image/upgrade.gif is also missing from the image directory. Adding an upgrade.gif does not enable upgrade in webmin configuration.
This was a new install of webmin/virtualmin with install.sh. This installed 1.690 webmin which was then upgraded to 1.700 by pointing software upgrade page to latest webmin. Recently it has automatically updated to 1.701.
If I turn off trusted referrer checking then I can run https://domain:1000/webmin/edit_upgrade.cgi, otherwise that URL just gives the warning.
If you installed Virtualmin, this is expected. When a new version of Webmin or Usermin is available to Virtualmin users, you will see a notification on the system information page after logging into Virtualmin.