1.删除自带的yum包,清除/etc/yum.repos.d下面的文件
rpm -qa|grep yum|xargs rpm -e --nodeps（不检查依赖，直接删除rpm包）
2.安装centos的yum
wget https://mirrors.aliyun.com/centos/6/os/x86_64/Packages/yum-metadata-parser-1.1.2-16.el6.x86_64.rpm
wget https://mirrors.aliyun.com/centos/6/os/x86_64/Packages/yum-3.2.29-81.el6.centos.noarch.rpm
wget https://mirrors.aliyun.com/centos/6/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.30-41.el6.noarch.rpm
wget https://mirrors.aliyun.com/centos/6/os/x86_64/Packages/yum-utils-1.1.30-41.el6.noarch.rpm
wget https://mirrors.aliyun.com/centos/6/os/x86_64/Packages/python-urlgrabber-3.9.1-11.el6.noarch.rpm
3.安装yum包
rpm -ivh python-urlgrabber-3.9.-.el6.noarch.rpm
rpm -ivh yum-metadata-parser-1.1.-.el6.x86_64.rpm
rpm -ivh yum-3.2.-.el6.centos.noarch.rpm yum-plugin-fastestmirror-1.1.-.el6.noarch.rpm (这两个包相互依赖 同时安装即可)
4.修改CentOS-Base.repo文件中的$releasever全部替换为版本号6或者7 根据自己的系统设置
5.生成缓存
yum clean all
yum makecache
yum update