vi /etc/yum.repos.d/upgradtool.repo

[upg]
name=CentOS-$releasever-UpgradeTool
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

yum -y install redhat-upgrade-tool preupgrade-assistant-contents

rpm --import http://mirrors.neusoft.edu.cn/centos/7/os/x86_64/RPM-GPG-KEY-CentOS-7

redhat-upgrade-tool --force --network 7 --instrepo=http://mirrors.yun-idc.com/centos/7/os/x86_64/

reboot