CentOS: from CentOS 6.5 to CentOS 7

https://wiki.centos.org/TipsAndTricks/CentOSUpgradeTool

$ cat /etc/redhat-release

CentOS release 6.8 (Final)

$ sudo yum -y install openscap pcre-devel libxml2-devel libxslt-devel

$ sudo yum -y install m2crypto python-simplejson

$ cd /etc/yum.repos.d

$ sudo vim upgradetool.repo 

[upg]

name=CentOS-$releasever - Upgrade Tool
baseurl=http://dev.centos.org/centos/6/upg/x86_64/
gpgcheck=1
enabled=1

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6

$ sudo yum install redhat-upgrade-tool preupgrade-assistant-contents

$ sudo su -

# preupg -s CentOS6_7

$ sudo reboot

$ sudo centos-upgrade-tool-cli --network 7 --instrepo=http://mirror.centos.org/centos/7/os/x86_64/