1.备份，将 CentOS-Base.repo 为CentOS-Base.repo.backup
    mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup

    2.下载新的 http://mirrors.aliyun.com/repo/Centos-7.repo,并命名为CentOS-Base.repo
    wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
    或者
    curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

    3.清除缓存
    yum clean all # 清除系统所有的yum缓存
    yum makecache # 生成yum缓存
    yum update

    查看yum源
    查看所有的yum源：

    yum repolist all
    查看可用的yum源：

    yum repolist enabled