redhat默认自带的yum源需要注册，才能更新，报错：
This system is not registered to Red Hat Subscription Management. You can use subscription-manager to register.
可替换为centos对应的源。 操作如下：
1.检查是否安装yum包。查看RHEL是否安装了yum，若是安装了，那么又有哪些yum包：
rpm -qa |grep yum
2 删除redhat自带的yum包
rpm -qa|grep yum|xargs rpm -e –nodeps（不检查依赖，直接删除rpm包）
rpm -qa |grep yum （查询确认）
[root@localhost ~]#
3.下载新的yum包。使用Centos6.5的yum包
1） 查看版本号和系统类别：
cat /etc/redhat-release
arch
2）根据上一步，找到对应的yum包，然后下载。我的服务器对应的为：
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-metadata-parser-1.1.2-16.el6.x86_64.rpm
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-3.2.29-60.el6.centos.noarch.rpm
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.30-30.el6.noarch.rpm
3）执行：
rpm -ivh yum-metadata-parser-1.1.2-16.el6.x86_64.rpm yum-3.2.29-60.el6.centos.noarch.rpm yum-plugin-fastestmirror-1.1.30-30.el6.noarch.rpm
如果这一步报错，大底如下：
libc.so.6 is needed by yum-metadata-parser-1.1.2-16.el6.i686
libc.so.6(GLIBC_2.0) is needed by yum-metadata-parser-1.1.2-16.el6.i686
libc.so.6(GLIBC_2.1.3) is needed by yum-metadata-parser-1.1.2-16.el6.i686
libglib-2.0.so.0 is needed by yum-metadata-parser-1.1.2-16.el6.i686
libpthread.so.0 is needed by yum-metadata-parser-1.1.2-16.el6.i686
libpython2.6.so.1.0 is needed by yum-metadata-parser-1.1.2-16.el6.i686
libsqlite3.so.0 is needed by yum-metadata-parser-1.1.2-16.el6.i686
libxml2.so.2 is needed by yum-metadata-parser-1.1.2-16.el6.i686
libxml2.so.2(LIBXML2_2.4.30) is needed by yum-metadata-parser-1.1.2-16.el6.i686
则说明下载安装的yum包与系统版本不匹配，需要重新下载安装。另外，163镜像站点对于6.0~6.5的资源均合并在6的目录下。
4）更换yum源，将原有源删除或备份到别的目下下：
cd /etc/yum.repos.d/
wget http://mirrors.163.com/.help/CentOS6-Base-163.repo
vi CentOS6-Base-163.repo
编辑文件，把文件里面的$releasever全部替换为版本号：6（注意，不是6.5！）最后保存！
4.清除原有缓存，重建缓存：
clean all
yum makecache
5.更新系统：
yum update
参考：
http://qingwang.blog.51cto.com/505009/1551228
http://www.th7.cn/system/lin/201402/50472.shtml
yum包下载：http://mirrors.163.com/centos/6/os/x86_64/Packages/
