The radio is off, but the USB provides power. To enter firmware update mode:

After extensive research across the following sources, no official firmware update file has ever been released:

Verdict: There is no way for an end user to update the firmware of a standard D-808.

If a user manages to obtain the firmware file (usually via a seller or repair shop), the update process is as follows:

Warning: Attempting to flash firmware intended for a different hardware revision will render the radio inoperable ("bricked").

This is the most critical aspect of this report: XHDATA does not release firmware files publicly for user installation.