Download and install Open TFTP Server from Sourceforge:
http://sourceforge.net/projects/tftp-server/?source=dlp
Once installed, copy the firmware image to the C:\OpenTFTPServer directory on your computer.
Next, run the TFTP Server:
Start > Programs > Open TFTP Server > Run Stand Alone
From the root prompt/command line of the Digi unit run the command to upload the firmware:
#> boot load=(tftp_host):(firmware_filename)