To flash the firmware, first put your board into bootloader mode by double clicking the reset button (either on the MCU board itself, or the one that is part of your keyboard). The controller should appear in your OS as a new USB storage device.
Once this happens, copy the correct UF2 file (e.g. left or right if working on a split), and paste it onto the root of that USB mass storage device. Once the flash is complete, the controller should automatically restart, and load your newly flashed firmware.