卸载：
1，查看yum是否已安装
[root@lob /]# rpm -qa|grep yum
yum-metadata-parser-1.1.2-16.el6.x86_64
yum-3.2.29-40.el6.centos.noarch
yum-plugin-fastestmirror-1.1.30-14.el6.noarch
2，删除centos原有的yum
[root@lob /]#rpm -qa|grep yum|xargs rpm -e --nodeps
安装：
操作系统是centos5.6 x86_64,不同的机器已安装的包有所区别，关键是掌握方法。
1下载yum的rpm安装包
[root@localhost rpm]# wget http://mirror.centos.org/centos/5.7/os/x86_64/CentOS/yum-3.2.22-37.el5.centos.noarch.rpm
2 [root@localhost rpm]# rpm -qa | grep yum //确定没有安装yum的rpm包
3 [root@localhost rpm]# rpm -ivh yum-3.2.22-37.el5.centos.noarch.rpm
error: Failed dependencies:
yum-fastestmirror is needed by yum-3.2.22-37.el5.centos.noarch
yum-metadata-parser >= 1.1.0 is needed by yum-3.2.22-37.el5.centos.noarch
需要安装依赖的包
4 找第一个依赖(下载）
[root@localhost rpm]# wget http://mirror.centos.org/centos/5.7/os/x86_64/CentOS/yum-fastestmirror-1.1.16-16.el5.centos.noarch.rpm
5 安装时发现依赖于yum，这个问题可以两个rpm包一起安装解决，先解决第二个依赖
[root@localhost rpm]# rpm -ivh yum-fastestmirror-1.1.16-16.el5.centos.noarch.rpm
error: Failed dependencies:
yum >= 3.0 is needed by yum-fastestmirror-1.1.16-16.el5.centos.noarch
6 先解决第二个依赖
[root@localhost rpm]# rpm -ivh yum-metadata-parser-1.1.2-3.el5.centos.x86_64.rpm
error: Failed dependencies:
libxml2.so.2()(64bit) is needed by yum-metadata-parser-1.1.2-3.el5.centos.x86_64
7 发现依赖，下载安装（我个人试验没安装这步，而且不懂为啥突然冒出这步）
[root@localhost rpm]# wget http://mirror.centos.org/centos/5.7/os/x86_64/CentOS/ libxml2-2.6.26-2.1.12.x86_64.rpm
[root@localhost rpm]# rpm -ivh libxml2-2.6.26-2.1.12.x86_64.rpm
Preparing... ########################################### [100%]
1:libxml2 ########################################### [100%]
8 第二个依赖解决，如果还是缺少包，继续下载安装。可以登录http://mirror.centos.org/centos/5.7/os/x86_64/CentOS/ 去找
[root@localhost rpm]# rpm -ivh yum-metadata-parser-1.1.2-3.el5.centos.x86_64.rpm
Preparing... ########################################### [100%]
1:yum-metadata-parser ########################################### [100%]
9 检验一下yum包的依赖情况
[root@localhost rpm]# rpm -ivh yum-fastestmirror-1.1.16-16.el5.centos.noarch.rpm
error: Failed dependencies:
yum >= 3.0 is needed by yum-fastestmirror-1.1.16-16.el5.centos.noarch
[root@localhost rpm]# rpm -ivh yum-3.2.22-37.el5.centos.noarch.rpm
error: Failed dependencies:
yum-fastestmirror is needed by yum-3.2.22-37.el5.centos.noarch
10 两个一起安装，解决
[root@localhost rpm]# rpm -ivh yum-3.2.22-37.el5.centos.noarch.rpm yum-fastestmirror-1.1.16-16.el5.centos.noarch.rpm
Preparing... ########################################### [100%]
1:yum-fastestmirror ########################################### [ 50%]
2:yum ########################################### [100%]
11 有需要可以yum -y update
转自：http://blog.csdn.net/gg296231363/article/details/7027668 略有添加修改
