For those of you who want to unroot or unbrick your T-Mobile Galaxy S2 SGH-T989 back to Android 2.3.6 for whatever reason (maybe bricked or want to take it back for warranty), here’s how to do it.
UPDATE: There’s an unroot method to ICS, please see How to Unroot/Unbrick to ICS on T-Mobile Galaxy S2 SGH-T989!
*Note – You will need a Windows-based computer, does not work with Heimdall on Mac or Linux.
Step 1. Plug-in a micro-USB cable from your phone to Windows computer. Then hold down the Volume Up, Volume Down, and the Power button for about 10 seconds.
Step 2. When your phone resets, only let go of the Power button.
Step 3. Once you see the warning screen, hit the Volume Up button to enter Downloading Mode.
Step 4. Download ODIN and stock ROM below. Unzip both and you should get the ODIN program and ROM in tar file format.