Forcing OTA Updates
From Android Wiki
How to force your G1 to install a downloaded OTA update
1: download the OTA Update zip file and save it to your sd card as "update.zip"
2: reboot your phone with the Boot Modifier home+power (hold home, press power, keep holding home)
3: press alt+s on the keyboard to apply the update.zip file (or press alt+l then alt+s to see details during the update)