I have few sites still running on CI 2.2.2. and wanna updates those to 2.2.3 but I don't know what are the modification files. So what files do I need to change in-order to upgrade form CI 2.2.2 to 2.2.3.
As stated here http://www.codeigniter.com/userguide2/installation/upgrade_223.html you need only to replace the system folder and index.php file with the new one:
Replace all files and directories in your "system" folder and replace your index.php file. If any modifications were made to your index.php they will need to be made fresh in this new one.