For those of you who want to unroot your Galaxy S2 i9100 to the latest ICS “official” stock ROM, you can do this easily using ODIN. (or Heimdall for Mac/Linux).
Step 1. Put your Galaxy S2 i9100 into download mode by holding down Volume Down, Center button, and Power button together for about 10 seconds until your phone reboots and you see the warning sign. Press Volume Up button to enter download mode.
Step 2. Connect a micro-USB cable to your Galaxy S2 from your computer.
Step 4. Download ODIN, unzip(Don’t have unzipping program? Download and install 7Zip.), and run the program.
Step 5. When ODIN runs, you should see a yellow highlight with a COM appearing. If you don’t see it, download Samsung Kies and install. Then plug-out and plug in your micro-USB cable to your phone, you should see it now.