备份源文件

cd /etc/yum.repos.d/

mkdir repo_bak

mv *.repo repo_bak/

下载源文件

curl -o /etc/yum.repos.d/CentOS-Base.repo filee.us/centos/7/CentOS-gulu.repo

清除缓存

yum clean all

yum makecache

更新yum,需要就更新不需要不用更新

yum -y update