How To Upgrade CentOS 6 to CentOS 7
Please do not attempt to upgrade from CentOS 6 to CentOS 7. The upgrade tool is deprecated by the CentOS Development Team and the process unlikely to succeed. Attempting to upgrade from CentOS 6 to CentOS 7 is highly likely to render your system inoperable.
For more information, please refer to the CentOS Wiki here.