How To Install / Flash Android 4.0.3 ICS On Unlocked Galaxy S II i9100
Step 1: Download the .zip specific to your particular device. Version I9100XWLP7 should be selected by those running a UK and Ireland purchased device, while those in Australia running on the Optus network should opt for build I9100XWLP8.
Step 2: Next, you’ll need to open Odin, which is included with the firmware file.
Step 3: Enable ‘USB Debugging‘ on your Galaxy S II by heading over to Settings > Applications > Development.
Step 4: Put your device in Recovery Mode by selecting Home + Volume up + Power.
Step 5: Next, select wipe data / factory reset. Then, in advanced, select wipe cache.
Step 6: Remove your battery from your device, then put it back in.
Step 7: Put your device into Download Mode by pressing Power + Volume down + Home simultaneously.
Step 8: In Odin, click PDA, then navigate to the extracted folder containing the downloaded Ice Cream Sandwich files and locate the file with the .tar.md5 extension.
Step 9: Then finally, click Start to begin the process.