清空源 & 备份：

sudo mkdir /etc/yum.repos.d/backup
sudo mv /etc/yum.repos.d/*.repo /etc/yum.repos.d/backup/

清空yum缓存：

sudo yum clean all

验证：

yum repolist

返回结果;

已加载插件：fastestmirror
Loading mirror speeds from cached hostfile
repolist: 0

更换阿里源

下载替换：

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

更新源：

yum clean all
yum makecache

验证：

sudo yum update -y