how to change the image on an item in unity code example

Example 1: set sprite unity

public Sprite sprite1; // Drag your first sprite here
public Sprite sprite2; // Drag your second sprite here

private SpriteRenderer spriteRenderer; 

void Start ()
{
    spriteRenderer = GetComponent<SpriteRenderer>(); // we are accessing the SpriteRenderer that is attached to the Gameobject
    if (spriteRenderer.sprite == null) // if the sprite on spriteRenderer is null then
        spriteRenderer.sprite = sprite1; // set the sprite to sprite1
}

void Update ()
{
    if (Input.GetKeyDown (KeyCode.Space)) // If the space bar is pushed down
    {
        ChangeTheDamnSprite (); // call method to change sprite
    }
}

void ChangeTheDamnSprite ()
{
    if (spriteRenderer.sprite == sprite1) // if the spriteRenderer sprite = sprite1 then change to sprite2
    {
        spriteRenderer.sprite = sprite2;
    }
    else
    {
        spriteRenderer.sprite = sprite1; // otherwise change it back to sprite1
    }
}

Example 2: unity change the source image or image

public Image test;
public Sprite example;

void Start()
{
 test.sprite = example; //change the source image of the image "test" to the sprite "example" 
}