1.备份旧的Yum源文件

cd /etc/yum.repos.d/
mv CentOS-Base.repo CentOS-Base.repo.backup

2.下载国内源

阿里云

curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

3.清理yum缓存

yum clean all && yum makecache

4.更新系统

yum update

5.验证yum源

yum repolist