Check out example codes for "how to change a image with code unity". It will help you in understanding the concepts better.

Code Example 1

// Change From GameOject
public GameObject GameObjectWithImage;
public Sprite ImageYouWant;
GameObjectWithImage.GetComponent<Image>().sprite = ImageYouWant

// Change From Image
public Image Image;
public Sprite ImageYouWant;
Image.sprite = ImageYouWant

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