镜像资源：
1. http://mirrors.163.com/
2. https://opsx.alibaba.com/mirror
从上列镜像资源下载如下rpm软件包
-rw-r--r--. 1 root root 39800 Jul 3 2014 python-iniparse-0.4-9.el7.noarch.rpm
-rwxr-xr-x. 1 root root 110540 Nov 20 2016 python-urlgrabber-3.10-8.el7.noarch.rpm
-rw-r--r--. 1 root root 1216680 Aug 10 2017 rpm-4.11.3-25.el7.x86_64.rpm
-rw-r--r--. 1 root root 107004 Aug 10 2017 rpm-build-libs-4.11.3-25.el7.x86_64.rpm
-rw-r--r--. 1 root root 281100 Aug 10 2017 rpm-libs-4.11.3-25.el7.x86_64.rpm
-rw-r--r--. 1 root root 82572 Aug 10 2017 rpm-python-4.11.3-25.el7.x86_64.rpm
-rwxr-xr-x. 1 root root 1288436 Aug 10 2017 yum-3.4.3-154.el7.centos.noarch.rpm
-rwxr-xr-x. 1 root root 28348 Jul 3 2014 yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
-rwxr-xr-x. 1 root root 32776 Aug 11 2017 yum-plugin-fastestmirror-1.1.31-42.el7.noarch.rpm
-rwxr-xr-x. 1 root root 120184 Aug 11 2017 yum-utils-1.1.31-42.el7.noarch.rpm
卸载已安装的yum组件
$ rpm -qa |grep yum (查看已安装)
$ rpm -qa |grep yum|xargs rpm -e --nodeps（不检查依赖，直接删除rpm包）
$ rpm -qa |grep yum (查看已安装)
安装CentOS 7 的yum 组件
$ rpm -Uvh rpm-*.rpm
$ rpm -ivh python-*.rpm yum-*.rpm
更新yum源
$ mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
$ wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
修改CentOS-Base.repo文件中的$releasever全部替换为版本号7
生成缓存
$ yum clean all
$ yum makecache
$ yum update
Congratulations!!!