10 steps to Upgrade CentOS from 6.6 to 6.8

Steps to Upgrade CentOS from 6.6 to 6.8

Step 1: 
          # Log on to the server

Step 2:
          # Take the required backup like DB backup,scripts etc.

Step 3:
          # verfiy the current version
               $ cat /etc/issue
                    CentOS release 6.6 (Final)
                    Kernel \r on an \m
              $ cat /etc/redhat-release
                   CentOS release 6.6 (Final)

Step 4:
          # Verify the current kernal version
               $ uname -a
                    Linux xxx 2.6.32-504.el6.x86_64 #1 SMP Wed Oct 15 04:27:16 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

Step 5: 
          # check for the updates and do clean
                $ yum check-update
                $ yum list updates
                $ yum clean all

Step 6:
          # Do the required package upgrade
                $ yum update glibc* yum* rpm* python*

Step 7:
          # Now Upgrade from 6.6 to 6.8 
               $ yum update
                   The above command will update all the yum files and along with kernel file.
Installed:
  kernel.x86_64 0:2.6.32-642.4.2.el6                                                                                                                                         
Complete!

Step 8:
          # Reboot the machine
          Once the upgrade completed,do the reboot
               $ sudo reboot

Step 9:
          # Verify the upgrade by checking the OS version
                $ cat /etc/issue
                     CentOS release 6.8 (Final)
                     Kernel \r on an \m

Step 10:
          # verify the kernel version
                $ uname -r
                     2.6.32-642.4.2.el6.x86_64

You can verify the centos kernel version in below link
https://access.redhat.com/articles/3078

Now you have successfully upgraded centos 6.6 to 6.8