Perhaps temporarily disable whatever is using the port, install Webmin, then change the port within Webmin, then re-enable the service using port 10000.
You only need to do that once. Once you install it, you can configure Webmin to listen on a different port, and then subsequent upgrades should be fine.
That said, I assume Webmin is an unconfigured state (because there’s no way for the package manager to know what port it uses, so it has to be the postinstall script that is failing), so it is already partially installed (the files are there but the postinst script failed). If so, you can edit /etc/webmin/miniserv.conf to change the port from 10000 to something else, and then run dpkg --configure -a to trigger the postinstall script again, which will try to start Webmin again and will presumably succeed.