#1、运行以下命令备份原软件源。
sudo mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
#2、运行以下任一命令添加CentOS 7软件源。
sudo wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

sudo curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

#3、运行以下命令生成本地缓存，加快搜索、安装软件的速度。
sudo yum clean all && sudo yum makecache

之后yum报错：
Errors during downloading metadata for repository ‘base’:

• Status code: 404 for http://mirrors.163.com/centos/3/os/x86_64/repodata/repomd.xml (IP: xx.xx.xx.xx)
  错误：为 repo ‘base’ 下载元数据失败 : Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried

看了一下配置文件:

官网的版本已经出到7以上了，当前还是3
里面有一些$releasever，将这些$releasever改为7

#修改repo文件里面的内容
sed -i 's/\$releasever/7/g' CentOS-Base.repo
#执行OK
sudo yum clean all && sudo yum makecache