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

[upg]name=CentOS-$releasever - Upgrade Toolbaseurl=http://dev.centos.org/centos/6/upg/x86_64/gpgcheck=1enabled=1gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6

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

rpm --import

redhat-upgrade-tool --force --network 7 --instrepo=

reboot