If you're talking about an upgrade from H/PC Pro, there is no such thing. You actually have to replace the physical ROM chip, on which the OS is permanently written and cannot be deleted or changed - and only two H/PC Pro devices' OEMs later offered H/PC 2000 ROM chips: the Intermec 6651 and the Fujitsu PenCentra 200.
(The NEC MobilePro 790 ROM chip can be used to upgrade a 780 or even an 880, though NEC never actually marketed the ROM chips separately. In the 880, the 790 ROM disables the USB port.
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