官方升级教程：http://wiki.centos.org/TipsAndTricks/CentOSUpgradeTool
1、在"/etc/yum.repos.d/"目录中创建文件upgradetool.repo
# cd /etc/yum.repos.d/
# touch upgradetool.repo
2、把以下内容贴进upgradetool.repo
# vi upgradetool.repo
文件内容如下：
[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
保存退出
3、安装升级工具
# yum install redhat-upgrade-tool preupgrade-assistant-contents
4、安装完后，运行
# preupg -l
查看支持的升级
5、执行升级分析
# 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
# rpm --import http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-7
7、升级系统
# 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
# centos-upgrade-tool-cli --network 7 --instrepo=http://mirrors.163.com/centos/7/os/x86_64/
8、重启系统，完成