This is sort of an old topic, but one of my first discoveries with this fone.

The first thing that happend, was that the 2.1-update1 was initially 'leaked'(straight off the garmin-asus site mind you.). Then after several weeks, the 'official' 2.1-update1 was released. I don't believe there's any difference in-between the two, at least I haven't noticed. Though when compressing to zip, both releases are different file sizes, varying only a few mbs. 

Here's the place where you'll find a download link and a 'How To'
http://www.garminasus.com/en_US/support/eclair/

They seem to move stuff around all the time and make changes, so here's the original 'How To' guide for upgrading your Garminfone.

A new software upgrade is available for the T-Mobile Garminfone that will upgrade the Android Operating System to Android 2.1 (Eclair). To upgrade your phone, please follow these steps:
Downloading the .update file to your computer

  1. Visit www.garminasus.com/en_US/support
  2. Click 'Software Updates'
  3. Download the 'Android 2.1 (Eclair) Update' for your Garminfone to your computer
    • Note:  Please save this file someplace easy to find, such as your Desktop
Copying .update file to Garminfone
  1. Connect your Garminfone to your computer using the USB cable provided in your box
  2. On the device, select "File Transfer" mode when the "USB connected" dialog appears
  3. On the computer, double-click on the Garminfone icon displayed on your computer
    • Windows Users:  You can find this icon in "My Computer" or "Computer" (usually located on your desktop or in your Start Menu)
    • Mac Users:  You can find this icon on your desktop
  4. Double-click the "Garmin" folder
  5. Double-click the "Update" folder
    • Note: If this folder does not exist, please right-click, select New Folder, and create a folder titled 'Update' (without quotes and with a capital U)
  6. Right-click the 006-B1130-00.update file at the location that you saved the file (e.g. on the Desktop)
    • Mac Users: Hold the Control key when clicking to "right-click"
    • Note: The file name must not be changed.
  7. Click Copy
  8. Right-click the Update folder, within the Garmin folder on the Garminfone
    • Mac Users: Hold the Control key when clicking to "right-click"
  9. Click Paste
  10. Once the transfer is complete, please Exit File Transfer Mode by sliding down the notification bar from the top of your phone's screen, touching USB File Transfer Mode, and touching 'Yes'.
  11. Remove your device from the computer.
    • Note:  It is important to safely remove the device from your computer in order for the file copy to be successful
Updating your device
Once you have ejected the Garminfone from your computer, the UpdateLauncher application will display.
Note:  Please ensure that the Garminfone is connected to a power source during the entire update process.
  1. When the UpdateLauncher application displays a prompt to apply the update, select "OK" and the update process will begin.
    • Note: If you see a message stating that the system version is the same as the system update file select, "Yes" to proceed.
  2. Leave device undisturbed until update is finished. The device will reboot itself into the update mode and then reboot to the lock screen.
  3. When you see the lock screen, your device will be updated.
After the phone restarts again, Garminfone’s Android 2.1 update will be complete.