CentOS7更新阿里云yum源，参考阿里云官方文档，操作如下：
1，备份旧源文件
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
2，下载新源文件
wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo
或者
curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo
3，清理旧缓存
yum clean all
4，生成新缓存
yum makecache