First login to your CentOS server as the root user. Execute the command "yum update -y" and you should see output similar to the following:

# yum update -y
Loaded plugins: fastestmirror
Determining fastest mirrors
* base: mirrors.kernel.org
* updates: mirrors.cmich.edu
* addons: mirror.stanford.edu
* extras: mirrors.usc.edu
base                                                     | 1.1 kB     00:00    
primary.xml.gz                                           | 878 kB     00:00    
base                                                           2508/2508
updates                                                  |  951 B     00:00    
primary.xml.gz                                           | 222 kB     00:04    
updates                                                        339/339
addons                                                   |  951 B     00:00    
primary.xml.gz                                           |  157 B     00:00    
extras                                                   | 1.1 kB     00:00    
primary.xml.gz                                           | 100 kB     00:00    
extras                                                         311/311
Setting up Update Process
Resolving Dependencies
--> Running transaction check
---> Package cups-libs.i386 1:1.3.7-8.el5_3.6 set to be updated
---> Package nash.i386 0:5.1.19.6-44.1 set to be updated
---> Package audit-libs.i386 0:1.7.7-6.el5_3.3 set to be updated
etc..etc..

Complete!

After the updates are complete execute "yum update -y" a second time to ensure CentOS is up to date. You should see the following on your screen if your system is up to date:

# yum update -y
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: mirrors.kernel.org
* updates: mirrors.cmich.edu
* addons: mirror.stanford.edu
* extras: mirrors.usc.edu
Setting up Update Process
No Packages marked for Update

After all updates have been applied it is recommended that you perform a soft reboot of your server using the "shutdown -r now" command from the shell prompt. This is to ensure you boot into any new kernels that YUM may have installed.