The current stable version of CentOS Linux OS is 6.5
If you want to upgrade your VPS / dedicated server to CentOS 6.5 from 6.4/6.3, follow below steps.
Login to your server as root via SSH and run following commands.
yum clean all yum update
This will update CentOS to 6.5 final.
After update check the CentOS version installed on your server:
Output should be CentOS release 6.5 (Final)