Communities like the FT-2000 Groups.io often host mirrors of older firmware versions and detailed "how-to" PDFs written by experienced users.
Press and hold the [GEN] + [5] + [0] keys while turning the power on to see your current firmware versions. Ft 2000 Upgrade Software
Note that there is a small "Programmer Switch" located behind a rubber cap or on the rear panel (depending on the specific sub-model) that must be toggled to the "ON" position before the software can write to the CPU. Where to Find the Software Communities like the FT-2000 Groups