For those who can't wait for the OTA update to be happen in your Nexus One, here are the manual steps to perform the upgrade:-
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. Enjoy!
Source:-
Intomobile