Flash the latest dualETH-PixelControl firmware to your device over USB, directly from this page.
Versions are pulled live from the project's GitHub releases.
Plug the device into USB, then click Connect and pick the serial port from the browser dialog. The board's USB-serial adapter usually appears as cu.usbserial-… (macOS) or COM… (Windows).
Erases the current firmware and writes the selected version. Takes about 30 seconds at 115200 baud.