after upgrading nexus s to 4.1.2, it’s not straight forward to downgrade to 2.3.6. A error occurs when fastboot -w update olderVersionAndroid.zip.
fastbooot flash recovery recovery-clockwork-126.96.36.199-crespo.img (which is downloaded from the Internet)
boot to bootloader, enter recovery mode, install rom (zip file in sdcard) from zip, which is downloaded from the Internet. http://forum.xda-developers.com/showthread.php?t=1445570
I’m not sure the rom is official rom as later there comes errors, A flashable rom is a zip file with boot.img, bootloader.img, radio.img and system folder, recovery folder and META-INF folder.
After that, the older version android is installed.
But when the system starts, there comes a lot of package crash errors and I’m not able to enter the system.
Then I flash the older version factory img with regular steps. It succeeds.
Ps: nexus S adb driver need to be downloaded from Samsung http://www.samsung.com/us/support/owners/product/GT-I9020FSTTMB,