方法一：
1. 配置yum.conf
终端输入以下命令
sudo chmod +w /etc/yum.conf
sudo gedit /etc/yum.conf
将yum.conf配置如下
[main]
cachedir=/var/cache/yum
debuglevel=2
logfile=/var/log/yum.log
pkgpolicy=newest
distroverpkg=redhat-release
tolerant=1
exactarch=1
[base]
name=Red Hat Linux $releasever - $basearch - Base
baseurl= http://centos.ustc.edu.cn/centos/5.4/os/i386/
[updates]
name=Red Hat Linux $releasever - Updates
baseurl= http://mirror.centos.org/centos/5.4/updates/i386/
然后保存退出。
2. 测试
在终端输入命令：yum update
看一下效果！
方法二：
1. 查看当前安装的YUM软件包并将其删除
查看
在终端里输入以下命令
rpm -qa|grep yum
屏幕上会出现你已经安装的YUM包
删除
rpm -e XXXXXXXXX(刚才显示安装的包) --nodeps
就可以将你已经安装的包都删掉了。
不信你可以再用第一步的命令看下。
2. 下载CENTOS的YUM包并将下载包安装
下载
从http://centos.ustc.edu.cn/centos/5.4/os/i386/CentOS/下载以下文件并安装
yum-fastestmirror-1.1.16-13.el5.centos.noarch.rpm
yum-3.2.22-20.el5.centos.noarch.rpm
yum-updatesd-0.9-2.el5.noarch.rpm
或者找个FTP站点用wget下载：
#wget ftp://ftp.pdc.kth.se/pub/bambi/centos/5.4/os/i386/CentOS/yum-fastestmirror-1.1.16-13.el5.centos.noarch.rpm
#wget ftp://ftp.pdc.kth.se/pub/bambi/centos/5.4/os/i386/CentOS/yum-3.2.22-20.el5.centos.noarch.rpm
#wget ftp://ftp.pdc.kth.se/pub/bambi/centos/5.4/os/i386/CentOS/yum-updatesd-0.9-2.el5.noarch.rpm
在终端进入你下载以上文件的文件夹,本人是/root/download/，并安装下载的包
#cd /root/download/
#rpm -ivh yum*.rpm
你可以看到安装进度。
3. 修改自己的CentOS-Base.repo
#cd /etc/yum.repos.d
#mv rhel-debuginfo.repo rhel-debuginfo.repo.bak （备份原来的配置文件）
建一个文件名字叫CentOS-Base.repo，然后配置如下：
# CentOS-Base.repo
#
# This file uses a new mirrorlist system developed by Lance Davis for CentOS.
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client. You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#
[base]
name=CentOS-$releasever - Base
baseurl=http://centos.ustc.edu.cn/centos/5.4/os/$basearch/
gpgcheck=1
gpgkey=http://centos.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-5
#released updates
[updates]
name=CentOS-$releasever - Updates
baseurl=http://centos.ustc.edu.cn/centos/5.4/updates/$basearch/
gpgcheck=1
gpgkey=http://centos.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-5
#packages used/produced in the build but not released
[addons]
name=CentOS-$releasever - Addons
baseurl=http://centos.ustc.edu.cn/centos/5.4/addons/$basearch/
gpgcheck=1
gpgkey=http://centos.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-5
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
baseurl=http://centos.ustc.edu.cn/centos/5.4/extras/$basearch/
gpgcheck=1
gpgkey=http://centos.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-5
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
baseurl=http://centos.ustc.edu.cn/centos/5.4/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://centos.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-5
注：如果http://centos.ustc.edu.cn/centos这个服务器的CentOS映像不够快的话用自己适合的服务器替换，可以用IE上去先找找看～
4. 导入KEY文件，注意大小写
rpm --import http://centos.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-5
或者
rpm --import http://centos.arcticnetwork.ca/5.4/os/i386/RPM-GPG-KEY-CentOS-5
5. 测试
#yum update
出来了～
[root@localhost download]# yum update
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
addons | 951 B 00:00
addons/primary | 201 B 00:00
base | 2.1 kB 00:00
base/primary_db | 1.6 MB 00:09
extras | 1.1 kB 00:00
extras/primary | 107 kB 00:00
extras 325/325
updates | 1.9 kB 00:00
updates/primary_db | 251 kB 00:01
Setting up Update Process
Resolving Dependencies
There are unfinished transactions remaining. You might consider running yum-complete-transaction first to finish them.
The program yum-complete-transaction is found in the yum-utils package.
--> Running transaction check
---> Package Deployment_Guide-en-US.noarch 0:5.2-11.el5.centos set to be updated
。。。。。。
