Some information about Electrodragon SPDT double relay board that is not on the wiki pages
Flashing the board
To be able to flash the board, the serial bitrate should be 9600, 57600 or 115200 baud. btn2 should be push on reset till beginning of flash (blue LED blink on the other side) after it could be released (keep it pushed till the end of flash if trouble).
Access to GPIOs
GPIO4 and GPIO5
On the main pin header, IO4 and IO5 are inverted thus IO4 is GPIO5 of ESP-12F ans IO5 is GPIO4.