1.备份/etc/yum.repos.d/CentOS-Base.repo文件

cd /etc/yum.repos.d/
mv CentOS-Linux-AppStream.repo CentOS-Linux-AppStream.repo.back

2.下载阿里云的Centos-6.repo文件

wget -O CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-8.repo
mv Centos-8.repo CentOS-Linux-AppStream.repo

3.重新加载yum

yum clean all
yum makecache