If Webmin was installed using the standard Webmin package (the RPM from Webmin.com), you can just run the install script. It’ll work fine.
If it was not installed using a standard package, you’ll need to uninstall it first. How it was installed will determine how you uninstall it (and I don’t know that information). If a non-standard rpm, use “rpm -e webmin”. If a tarball install, run the uninstall.sh script in the /etc/webmin directory. (Actually, it looks like Webmin is smart enough to know how it was installed, and so running /etc/webmin/uninstall.sh will always do the right thing.)