1. 备份
2. 安装依赖列表
yum源文件/etc/yum.repos.d/upgrade.repo，内容为
[upgrade] name=upgrade baseurl=http://dev.centos.org/centos/6/upg/x86_64/ enabled=1 gpgcheck=0
安装依赖包
yum install preupgrade-assistant-contents redhat-upgrade-tool preupgrade-assistant
3. 校验升级
执行刚刚安装的命令
preupg
导入yum的GPG密钥
rpm --import http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-7
4. 开始升级
执行升级命令
/usr/bin/redhat-upgrade-tool-cli --force --network 7 --instrepo=http://mirror.centos.org/centos/7/os/x86_64
可以换成阿里云的镜像，速度会快一些
/usr/bin/redhat-upgrade-tool-cli --force --network 7 --instrepo=http://mirrors.aliyun.com/centos/7/os/x86_64/
如果遇到报错
Error: database disk image is malformed
清除缓存，再次重试
yum clean dbcache
5. 重启
reboot