Change the hostname line in /etc/sysconfig/network to the hostname you want (this file may be different on FreeBSd and others). Your hostname should be something prefixing a domain you own (this is referred to as a fully qualified domain name, FQDN), so server1.yourdomain.com for example, not just yourdomain.com. Changing it in that file ensure that your hostname gets set each time networking starts, however you'd need to restart networking for it to get set in the current session with just that changed. This is not required for our current running environment however, we can just set it with the hostname command. To do so run:
If you are running a cPanel server, you will want to simply change the hostname in under both Main » Server Configuration » Basic cPanel/WHM Setup and Main » Networking Setup » Hostname.