I can't update my Nexus S, shows error screen. What to do?
When you see cute robot with the red triangle, press power + volume up together to get to the recovery menu, and select "update from /cache" or something similar you'll see there. Then the update process starts and, most probably, gives you some error messages about what's wrong with your phone.

If you have ever changed any system files, update process might require you revert them to the original condition before upgrade, otherwise patches cannot be applied properly.