There are 4 steps in a hostname change, luckily all the steps are easy.
Open the /etc/sysconfig/network file with your favorite text editor. Modify the HOSTNAME= value to match your FQDN host name.
# sudo nano /etc/sysconfig/network
Change the host that is associated to your main IPaddress for your server, this is for internal networking (found at /etc/hosts):
The 'hostname' command will let you change the hostname on the server that the commandline remembers, but it will not actively update all programs that are running under the old hostname.
At this point all the necessary changes that needed to be made have been made, you will want to restart networking on your server to make sure that changes will be persistent on reboot:
# /etc/init.d/network restart