You can easily upgrade to newest CentOS version if you are use a dedicated server or Xen VPS,KVM VPS,HVM VPS. But you can not upgrade if your VPS is based on OpenVz

How upgrade CentOS 6.3 to 6.4/6.5/6.6 or from CentOS 5.x to 5.9? 

1. Make sure your server is a dedicated server or Xen VPS,KVM VPS,HVM VPS
2. Backup data 
3. SSH login via root and run command "yum -y upgrade"
4. Wait 10-60 minutes, done!
5. Check CentOS version by command "cat /etc/redhat-release"

NOTE:

1. You can not upgrade from CentOS 5.x to 6.x
2. We recommand you upgrade ASAP after reload OS if you want try new version
3. There are no need to upgrade to newest version if all your software already works fine.