Now Google is rolling out the Android 2.2 update for the nexus one, Now the manual update is available from Google servers for Nexus one. The safe way to get your Nexus One updated to Android 2.2 is to wait for your phone to notify you that the over the air update is available, Do the manual update at your own risk.
This works only on a Nexus one running ERE27 firmware Android 2.1 and should not be rooted or running any mod.
1. Download the update zip file from Google.
2. Rename the file to update.zip.
3. Copy the update.zip file onto your microSD card.
4. With your Nexus One off, hold down the trackball and press the power button.
5. You’ll be booted into a white screen with three Android robots on skateboards. Select “Bootloader.”
6. On the next screen, select “Recovery.”
7. Your phone will reboot, giving you a picture of the Android robot and an exclamation point inside a triangle.
8. Now press the power button and volume up button at the same time. It could take a couple of tries.
9. Now (using the trackball this time) choose “Apply sdcard:update.zip” and let things run their course.