We do recommend a fresh installation of the operating system and cPanel if at all possible, but the instructions cPanelDavidG gave are valid. Specifically, use vi or nano to edit /etc/cpupdate.conf and make the CPANEL= line look like this:
Save the changes to /etc/cpupdate.conf and run the following command:
cPanel will update itself to the latest full RELEASE version.
Note that you will need a valid license upon upgrading to the full version. Either acquire a trial license or purchase a full license before updating to the full cPanel version from DNS-only.