Unity 5 change source image of UI image c# script

I am making a game, and in the level selector it has 3 stars for each level with different IDs. How do I change the source image for each star, because the stars are originally grey, but once a player does well enough, the player gets more stars. Thx!

Try image.sprite = [your sprite]

doesnt works for me, unity ignors this line and shows no error about it, the image stays with “none” sprite.
how can I fix this?

here is some of my code:

    private Image GetFace(Sprite player_f)
    {
        Image face = PlayerBarExample.GetComponentInChildren<Image>();
        face.sprite = player_f;
...
}