1）查看升级前的版本信息 lsb_release -a
LSB Version: :base-4.0-amd64:base-4.0-noarch:core-4.0-amd64:core-4.0-noarch:graphics-4.0-amd64:graphics-4.0-noarch:printing-4.0-amd64:printing-4.0-noarch Distributor ID: CentOS Description: CentOS release 6.8 (Final) Release: 6.8 Codename: Final
2）在“/etc/yum.repos.d/”目录中创建文件upgradetool.repo
[root@openstack ~]# cd /etc/yum.repos.d/
[root@openstack yum.repos.d]# touch upgradetool.repo
把以下内容贴进upgradetool.repo
[root@openstack yum.repos.d]＃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
3）安装升级工具
[root@openstack ~]# yum install redhat-upgrade-tool preupgrade-assistant-contents
4）安装完后，运行
[root@openstack ~]# preupg -l
CentOS6_7
5）执行升级分析
preupg仅仅是分析系统而不会执行任何任务，执行后它会给出一些建议告诉更新程序在更新的时候去搜索什么。
[root@openstack ~]# preupg -s CentOS6_7
Preupg tool doesn't do the actual upgrade.
Please ensure you have backed up your system and/or data in the event of a failed upgrade
that would require a full re-install of the system from installation media.
Do you want to continue? y/n
y
直到完成（跑相当长的时间，请耐心等待）
如果没有提示出错，可以继续下一步。否则请检查！！！要全部PASS才可以。
6）导入centos7源
[root@openstack ~]# rpm --import http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-7请看文章底部错误
7）升级系统
[root@openstack ~]# **centos-upgrade-tool-cli --network 7 --instrepo=http://mirror.centos.org/centos/7/os/x86_64/** 国内比较慢，可以使用以下源 [root@openstack ~]# centos-upgrade-tool-cli --network 7 --instrepo=http://mirrors.163.com/centos/7/os/x86_64/8）重启系统
[root@openstack ~]# reboot
重启系统，再次登录后就回发现系统已经升级到centos7了！
转自：https://www.cnblogs.com/kevingrace/p/5709571.html
Install Old Version OpenSCP
因为如果不重新装旧版本的openscp就会提示错误：preupg: error: [Errno 2] No such file or directory: '/root/preupgrade/result.html'
参看错误说明
yum erase openscap
yum install http://dev.centos.org/centos/6/upg/x86_64/Packages/openscap-1.0.8-1.0.1.el6.centos.x86_64.rpm
yum install redhat-upgrade-tool preupgrade-assistant-contents
导入RPM KEY升级
这里不可以导入最新的，因为最新版的.treeinfo中没有upgrade项了，所以这里导入7.2的。
rpm --import http://vault.centos.org/centos/7.2.1511/os/x86_64/RPM-GPG-KEY-CentOS-7
centos-upgrade-tool-cli --network 7 --instrepo=http://vault.centos.org/centos/7.2.1511/os/x86_64/
redhat-upgrade-tool --force --cleanup-post --network 7.0 --instrepo=http://vault.centos.org/centos/7.2.1511/os/x86_64/
Finally
清理el6的rpm包 rpm -qa | grep -i el6 ，具体删什么升级什么，视个人情况。 el6依赖包版本降级 yum downgrade grep
**更多请参考:https://wiki.centos.org/TipsAndTricks/CentOSUpgradeTool **