I am making day and night cycle in my game but i want to use standard material to add that metalic look on my plane but when it’s dark my plane is glosy how can i change this 
You can set the “metallic” value of your shader directly from your code, depending on whether it is night or day etc.
To set the value it should be something like:
this.GetComponent<Renderer>().material.SetFloat("_Metallic", someFloatValue);
Sorry I don’t have Unity right now I can’t check if “_Metallic” is the correct name. To verify it you can click on the top right of the shader “edit shader” (or something like that) and you should see correct shader’s code. The names will be at the top of the file.
For more information: