The .UPD file is an image of the firmware that runs on the datalogger. It can be downloaded in ASCII (.UPD) or as a compressed archive file (.ZIP). The Firmware Updater program for Windows (below) is used to write this file to the program memory on the circuit board.
The updater program writes the UPD firmware file to the datalogger circuit board over the serial port. The file is available as in installer (.zip) or just the executable (.exe). If you use the .exe and get a message about missing files, use the installer.