1. Download the Nexus One update from this link here.
2. Rename the file to update.zip (If your OS hides file extensions by default, rename the file to update to avoid it being renamed to update.zip.zip)
3. Connect your Nexus One to your computer and transfer the file to the root of the microSD card (inside no folders)
4. Power down the Nexus One. Now reboot it by holding the Power and Volume Down at the same time.
5. You should now see a screen with Androids on skateboards. Using the Volume keys, navigate to Recovery and press Power.
6. The recovery screen will only show a triangle with an exclamation point in it. To get to the recovery menu press Power and Volume Up at the same time. (It can be a bit tricky sometimes, so keep trying if it doesn’t work the first time)
7. Using the trackball, scroll down and select “Apply sdcard:update.zip.” to start the update.
8. Once the update has completed, reboot your Nexus One.
You’ve successfully updated to Android 2.3.3. Have fun!