In Centos 5.x it is very easy to update Kernal Online. Simply we have to login as root and issue following command

First Check Kernels available for update

yum list kernel-*

Then issue following command to install new kernel

yum install kernel-xxxx-yyyyy

It will install new kernel. We have to edit grub.conf to boot from this updated kernel

You can also update you kernel with rpm package manger. Just download kernel what you want and issue this command to install new kernel.

 rpm -ivh kernal-xxxx-yyyy.rpm

The following command can also be used to update kernel as well as kernel libraries

 yum update kernel-* 

I hope you will like this how-to.