Replacing an object depending on its health?

So I want to change the sprite of an object to show its health. I have a few sprites , all of a blue block, but with varying levels of cracks. I have no idea how to replace the sprites depending on health, can someone help? My current code is:

if(gameObject.tag == "green"){

			Destroy (gameObject);
		}
		if (gameObject.tag == "blue") {
			if (blockHealth != 1) {
				blockHealth++;
			} else {
				Destroy (gameObject);
			}
			if (blockHealth == 1) {
				SpriteRenderer.sprite = brick_VERY_damaged_blue;
			}
		}
		if (gameObject.tag == "yellow") {
			if (blockHealth != 2) {
				blockHealth++;
			} else {
				Destroy (gameObject);
			}

and so on for stronger blocks. However, the part where I have SpriteRenderer throws an error when I try to run the script, saying an object reference is required for the non-static field, method, or property ‘UnityEngine.SpriteRenderer.sprite.get’, and The name ‘brick_VERY_damaged_blue’ does not exist in the current context.

SpriteRenderer.sprite = brick_VERY_damaged_blue;
=>
GetComponent().sprite = brick_VERY_damaged_blue;

I don’t know what type “brick_VERY_damaged_blue” is, but if it’s of type Sprite, it should work.