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Uses Lets Control It/ESP Easy.
Download the latest ESP Easy Mega release.
Install esptool.py. Hook up a USB-to-serial adapter to the board.
Short _PROG & GND and press RESET.
Find the right firmware file (normal_ESP8266_4M1M) and run:
$ esptool.py --port /dev/cu.usbserial-14120 --baud 115200 write_flash 0 ESP_Easy_mega_20200516_normal_ESP8266_4M1M.bin