If you want to start testing CentOS 6.9 without the official release is there (re-packaging to fit all on one DVD) then install the CentOS CR (Continues Release) repository by: yum install centos-release-cr. After this, yum update will install CentOS 6.9.

Installation instructions for CentOS 7: The repository configuration file is included in the newest centos-release package. First update your system with yum update to get the new centos-release package, then run yum-config-manager –enable cr to enable the CR repository.