To upgrade to a new package use
-U and the RPM package file name:
rpm -U emacs-20.0-1.i386.rpm
This will install the new package, and uninstall any older emacs packages. If there were configuration files that you modified in the old package, your files will be saved to backup names, and RPM will notify you. If this happens you should investigate your configuration file and the new one, and perhaps incorporate your changes into the new version.