In place Upgrade CentOS 6.5 to 7.0 how to.
Is there an how to, what is your experience?
If it's time for a new OS, then it's time to get rid of all the old chuff, make sure your baremetal rebuild process is up to date, and do a clean install.
In place updates are nothing but trouble - suitable only for lazy sysadmins and noobs.
With RH6->RH7 there is a supported migration path.
This process isn't yet ready for CentOS 7. See