Download the version that you want from Releases.

Extract the zip file to your desired location. 

Open the "esp_os.ino" from the extracted zip. All required files are in the "src" folder. Here's a tutorial if you haven't installed the ESP8266 library yet. Select your board port for example I have COM4 and then click upload. Done! Don't forget to set the serial monitor baud rate to 115200 or it won't function properly.