In general we would like to discourage you changing hostnames! Various medium- and long-term problems with already hosted domains (on that server) and DNS setups and emails can be the unpleasant result of such a hostname change.
To change your hostname, enter the following command as root:
$ hostname servername.yourdomain.com
However, if you are using cPanel, (a) you better do this from within your WHM control panel, in the"Network Setup" section, "Change Hostname," and (b) you will need to resync your license with the cPanel license server. To do this, use the following command:
$ /usr/local/cpanel/cpkeyclt