Open an SSH session to your server and edit the following file (with nano, or vim, or any other editor)
sudo nano /etc/sysconfig/network
In that file, change the HOSTNAME value.
GNU nano 2.0.9 File: /etc/sysconfig/network NETWORKING="yes" GATEWAYDEV="venet0" NETWORKING_IPV6="yes" IPV6_DEFAULTDEV="venet0" HOSTNAME="example.org"
Save the file.
Now check your hostname by typing hostname in the command line. CentOS should output your current hostname.
Perhaps you might have to restart networking on your CentOS Linux server.