How do i know what build an firmware im using?
The firmware version( or Build Number) can be seen at Settings > About phone > Build number

IMPORTANT: This is only for the U.S G1 make sure the battery is fully charged or atleast more than 50%.

Steps:

1.Download from here MEGAUPLOAD - The leading online storage and file delivery service

2.Rename the archive to update (when you rename it make sure there is no .zip extension it should say just update)

3.Copy the file to the SD Card via usb

4.Turn off the phone. Hold the HOME button while turning on the phone. The phone will start in bootloader mode. You should see an exclaimation sign when it starts.

5. Press Alt + L, it will take you to the boot menu,

6. Press Alt + S, to start the update process. It will take a while so be patient.

7. When the update completes, it will prompt you to press HOME + BACK to reboot.

8 . After you reboot, the update will finalize and automatically reboot.

Thats it !

This should work for you i did it last night an it worked perfect,takes about 10-15 mins to do.