I had Galaxy S2 with Gingerbread kernel, I needed to upgrade it to ICS. I used Odin 3 v3.04 to download the ICS kernel (I9100) to the phone. I followed the following steps:
You start with the phone disconnected from the computer and switched off.
Start the SGS2 in download mode: you do this by pressing "Volume down", the "Home" button and the "Power" button at the same time. Don’t release them until the download mode screen appears (it looks totally different from the normal starting screen).
Follow the on-screen instructions: 'Continue' by pressing "Volume up".
Start Odin 3 on your computer.
Click on PDA and select the tar-archive containing the kernel you want to install.
DO ONLY check the option boxes "Auto Reboot" and "F. Reset Time".
Connect the SGS2 to the PC via USB. The first field of "ID:COM" should turn yellow stating the COM Port of your device. Continue only after this has happened. It can take a couple of seconds.
Startand wait for the flashing process to finish.
After that it said "Reset", and I powered off the phone
10.When I switched on the phone Odin said Pass
*But now I have a yellow triangle with an exclamation mark in the middle, no switch on happening, not able to get into download mode also.*