To change your centos hostname:
sudo vi /etc/sysconfig/network
Modify HOSTNAME value with your FQDN host name
HOSTNAME=myserver.domain.com
Than you should change host name which is associated with your IP
sudo vi /etc/hosts
127.0.0.1 localhost localhost.localdomain
#change ip 123.123.123.123 to your IP
123.123.123.123 hostname.mydomain.com hostname
At this point you should restart network service. You can do that with following command:
sudo /etc/init.d/network restart