yum配置文件所在路径：
/etc/yum.repos.d/
163镜像：
CentOS7
wget http://mirrors.163.com/.help/CentOS7-Base-163.repo
CentOS6
wget http://mirrors.163.com/.help/CentOS6-Base-163.repo
CentOS5
wget http://mirrors.163.com/.help/CentOS5-Base-163.repo
阿里云镜像：
CentOS7
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
CentOS6
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo
CentOS5
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-5.repo
下载镜像后，yum makecache，就可以使用镜像更新系统。