NodeMCU flasher is a firmware programmer for NodeMCU DEVKIT.
You can Download ESP8266 Flasher Here .
You MUST set GPIO0 to LOW before programmingand this is perform by pressing the flash button in NodeMCU DEVKIT, if there is no flash button you have to manually make the GPIO0 to low by connecting the wire.
You can download the firmware by here
You can also download the latest  firmware by here
The following steps to be followed while flash the NodeMCU DEVKIT.

1.Download the firmare and save it to a location.
2.Download the ESP8266 Flasher ESP8266 Flasher
3.Open the ESP8266 Flasher and click "Config" and click the "gear" icon and select the loaction of the firmware that you already downloaded.

Available link for download