How to Manually Install Android 2.3.3 On Nexus One
Step 1: Download Android 2.3.3 and save it on your desktop – Link posted belowCheck the link below to read for information to successfully update your Nexus One's OS.
Step 2: Rename the downloaded file to update.zip
Step 3: Now connect your Nexus One to your computer and Copy update.zip over to the root of your SD Card
Step 4: Now power off your Nexus One and reboot it by holding the Power and Volume Down at the same time.
Step 5: You should now see a screen with Androids on skateboards. Using the Volume keys, navigate to Recovery and press Power.
Step 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. (This part is little tricky. So, keep trying it)
Step 7: Using the trackball, scroll down and select “Apply sdcard:update.zip.” to start the update.
Step 8: Once the update has completed, reboot your Nexus On