备份

mkdir /etc/yum.repos.d/repo_bak/
mv /etc/yum.repos.d/*.repo /etc/yum.repos.d/repo_bak/

下载新的CentOS-Base.repo

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

清理缓存、创建缓存

yum clean all
yum makecache

查看启用的yum源和所有的yum源

yum repolist enabled
yum repolist all

更新yum

yum -y update