Prepare your ADB adapter by entering bootloader mode: with your ADB keyboard attached to the ADB adapter, press and hold the B key while plugging it into your computer. You may release the B key now.
Run QMK Toolbox. Choose the ATmega16u2 MCU. Click “Open” to browse for the firmware you unzipped. There are two firmware files: firmware_A.hex and firmware_B.hex. Choose one, and if it doesn’t work, try the other.