[1] 首先备份/etc/yum.repos.d/CentOS-Base.repo

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

[2] 进入yum源配置文件所在文件夹

cd /etc/yum.repos.d/

[3] 下载163或者阿里云的yum源配置文件，放入/etc/yum.repos.d/(操作前请做好相应备份)

163：

wget http://mirrors.163.com/.help/CentOS6-Base-163.repo

centos5：

wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-5.repo

centos6:

wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo

centos7:

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

[4] 运行yum makecache生成缓存

yum makecache

[5] 更新系统

yum -y update

参考链接：

centOS更改yum源与更新系统

超简单将Centos的yum源更换为国内的阿里云源