1.cat /etc/redhat-release
2.yum update
3.cat /etc/redhat-release，此时发现版本变成了 7.9.2009，怎么才能升级到8.x呢？
4.参考这篇文章：404-对不起！您访问的页面不存在，执行如下命令：
yum -y install epel-release
yum -y install rpmconf yum-utils
rpmconf -a
yum -y install dnf
dnf -y remove yum yum-metadata-parser
dnf -y upgrade
dnf -y upgrade http://mirrors.163.com/centos/8.0.1905/BaseOS/x86_64/os/Packages/centos-release-8.0-0.1905.0.9.el8.x86_64.rpm
dnf -y upgrade https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
dnf clean all
rpm -e --nodeps `rpm -q kernel`
dnf -y --releasever=8 --allowerasing --setopt=deltarpm=false distro-sync
