How to install android 4.0.3 ice cream sandwich for galaxy s2

Sporting a dual-core 1.2 GHz processor and 1 GB of RAM, the Galaxy S2 is certainly a worthy device capable of yet another upgrade.  Taking its roots from the previous Android 4.0.3 ICS XXKP8 firmware upgrade, the improved version features various bug fixes from the latter.

For those of you who can’t wait for the official ICS upgrade to be released, then you can try the Android 4.0.3 XXKPA upgrade, but you will need to manually update your phone.  If you installed the XXKP8 firmware earlier, you can also update to this version  because firmware XXKPA reportedly fixes many of the bugs and issues in the earlier firmware version.

Although firmware XXKPA still has some bugs and issues, users have reported that most of the apps seem to work in this version.  Touch response and the user interface are also reportedly faster and smoother.  Battery life is also reportedly longer with this build, and it is more stable than the previous build.

Important Notes
The instructions in this guide are known to work for the international variant (handset model GT-I9100) of the Galaxy S2 only, but not for the U.S. variants (such as the GT-I9100G).  You can check whether you have the I9100 model by dialing *#1234# on your phone and checking the model number.
This guide is also known to work for devices that are not network-locked.  If your device is network-locked, the instructions may not work.
If you’ve rooted your Galaxy S2 previously, you will lose root access after upgrading to firmware XXKPA.
Make a complete backup of your current working system before updating your phone.
Keep the phone at full charge, or with at least 70% of battery power.
Instructions
Download the Android 4.0.3 ICS package that contains the needed files for upgrading your device such as the Odin application and other essential firmware files.  Get the package here.
Extract the contents of the compressed package into a single folder on your computer.  (Use software for extracting RAR archives, such as WinRAR.)
Run Odin.exe file on your computer.
Enable USB debugging mode on your device by heading to “Settings > Applications > Development > USB Debugging”. Make sure that there is a check mark beside USB Debugging mode.
Turn off your phone and reboot it into Download mode. You can enter Download mode by simultaneously pressing down the Volume Down and Home button keys then pressing the Power button.
Once the device is already in Download mode, connect your device via USB cable to your computer while keeping the Odin app opened.
After a short while, the ID: COM section of the application will eventually turn yellow.  This means that the drivers for your device have been successfully installed and your device has been recognized by your computer.
In the Options section in the Odin application, make sure that only  “Auto Reboot”, and “F. Reset Time” are checked.
In the Odin application, click the tabs (specified in bold text below) and locate their associated extracted files that you have downloaded earlier.
PIT – u1_02_20110310_emmc_EXT4.pit
PDA –  I9100_CODE_I9100XXKPA_CL51925_REV02_user_low_ship.tar.md5
Phone – MODEM_I9100XXKPA_REV_02_CL1094591.tar.md5
CSC – GT-I9100-MULTI-CSC-OXAKPA.tar.md5
Double check that you’ve loaded the correct files into their respective tabs.  If you load the wrong file into a tab, you could brick your phone.  So, double check.
After all the corresponding files have been matched, click the “Start” button in Odin for the installation process to begin. Once the firmware has been successfully installed, your device should reboot itself.  Allow it to reboot completely.
Disconnect your phone from your computer.
Turn off your phone.
Reboot it into Download mode again.
Connect your phone to your PC via USB cable.
Assuming that you didn’t close the Odin.exe application from the previous steps, click on the PDA button and select “I9100_KERNEL_I9100XXKP9_CL45181_REV02_eng_mid_ship.tar.md5″.  Make sure that only this file is loaded.  The other tabs (e.g., PIT, CSC, Phone) should not show any file loaded.
Make sure that only “Auto Reboot” and “F. Reset Time” are checked in the Options section.
Press the Start button in Odin to begin installing the new kernel. Once the installation is done your phone should reboot itself.
Congratulations! You have successfully installed Ice Cream Sandwich on your Samsung Galaxy S2.