I. Before You Begin
1. This will return the phone back to the stock ROM and put you at build GRH78.
2. Use this method to start over, or to send your phone in for warranty reasons.
3. You MUST have a custom recovery image to use this unrooting method.
4. This WILL WIPE everything on your phone.
II. Create a Nandroid Backup
1. Turn off your phone.
2. Turn it back on by holding down Volume Up and Power until the green Android shows up on the phone.
3. Using the volume buttons and power to select, select Recovery from the menu.
4. Once in recovery mode, select Backup and Restore then select Backup and wait for it to backup your current ROM.
5. Once it is done, select Reboot and wait for the phone to reboot.
III. Flash the Stock Nandroid Backup
1. Download the stock Nandroid Backup and save it to your computer.
2. Extract the .zip file somewhere on your computer.
3. Plug your phone in via USB cable and mount the internal storage so we can transfer files to it.
4. Find the Clockword folder on your phone’s internal storage and open it, then open the Backup folder inside there.
5. Copy the 2010-12-22.21.06.07 folder from the .zip file you extracted and put it inside the Backup folder we just opened.
6. Turn off the phone.
7. Turn it back on by holding down Volume Up and Power until the green Android shows up on the phone.
8. Using the volume buttons and power to select, select Recovery from the menu.
9. Once in recovery mode, select Backup and Restore then select Restore and choose the 2010-12-22.21.06.07 backup and wait for it to restore the phone to stock.
10. Once it is done restoring, select Reboot and wait for the phone to reboot.
11. Once it does reboot, turn it off and then back on normally again (this is to clear out the recovery image and bring that back to stock as well).
IV. Relock the Bootloader
1. On the phone, go through the setup process clicking skip where ever possible.
2. Then hit Menu then Settings then Applications then Development and make sure USB debugging is checked ON.
3. Plug the phone in via USB.
4. Open a command prompt on the computer by clicking on the Start menu and typing cmd and hitting enter in the search bar.
5. Then in the command prompt, type the following with hitting enter at the end of each line (this is assuming you did our How To Root the Nexus S procedure):
adb reboot bootloader
fastboot oem lock
6. The phone should relock the bootloader and you are now fully stock and completely ready to send the phone in for warranty or go back to the How To Root procedure and reroot it.