Upgrade RHEL from 6 to 7

Enable repositories

subscription-manager repos --enable rhel-6-server-extras-rpms

subscription-manager repos --enable rhel-6-server-optional-rpms

Install required packages

yum install preupgrade-assistant preupgrade-assistant-contents

Perform the system assessment

preupg

Perform the upgrade

redhat-upgrade-tool --iso /tmp/rhel-server-7.3-x86_64-dvd.iso

reboot