Use the official Baofeng CPS to read your radio and save your codeplug (channels, contacts). OpenGD77 uses a different codeplug format, but you can export your channels as CSV.
You cannot use Baofeng CPS anymore. Download and install the OpenGD77 Codeplug Programmer. This software is vastly superior to the stock Baofeng CPS.
Run the loader. It will detect your radio in DFU mode. Select the OpenGD77 firmware file (for the DM1702, not the GD-77). Click "Write to Flash." The loader will first erase the stock firmware, then write the new one.