1备份：

cp /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d//CentOS-Base.repo.ori

2下载：

wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo

3：清理

yum clean all

4：更新缓存

yum makecache
yum install tree -y
yum update -y