Step 1: Download Android 2.3.6 Gingebread (GRK39F). [Direct Download]
Step 2: Rename the file to update.zip.
Step 3: Connect your Nexus One to your computer and transfer update.zip to the root of your phone’s microSD card, it should not be inside any folder.
Step 4: Turn off your Nexus One.
Step 5: Keep the Volume Down button pressed and turn on the phone. Scroll down to recovery and press the power/lock button.
Step 6: You should now see a triangle with an exclamation mark. Press Power + Volume Up button.
Step 7: A menu will appear. Select sdcard:update.zip. The screen will say install from sdcard complete.
Step 8: You must now select reboot system now.