1. 下载安装wget

yum install -y wget

2. 备份默认的yum

mv /etc/yum.repos.d /etc/yum.repos.d.backup

3. 设置新的yum目录

mkdir -p /etc/yum.repos.d

4. 下载阿里yum配置到该目录中，选择对应版本

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

5. 更新epel源为阿里云epel源

mv /etc/yum.repos.d/epel.repo /etc/yum.repos.d/epel.repo.backup 
mv /etc/yum.repos.d/epel-testing.repo /etc/yum.repos.d/epel-testing.repo.backup

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

6. 重建缓存

yum clean all 
yum makecache

7. 看一下yum仓库有多少包

yum repolist
yum update