1、备份

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

2、下载CentOS-Base.repo 到/etc/yum.repos.d/

cd /etc/yum.repos.d/
wget http://mirrors.163.com/.help/CentOS7-Base-163.repo

3、生成缓存

yum clean all
yum makecache

4、更新系统

yum update