Changing Text within button

I am trying to set the text of a button via c# script. I have checked out other answers, but they do not help. I have tried this multiple times:

button.GetComponentInChildren<GUIText>().text = "Button";

But I always get the error “Object reference not set to an instance of an object”. How can I change the text of the button?

If it is a button from the new Unity UI system, use:

using UnityEngine.UI;

//Later on in program
        button.GetComponentInChildren<Text>().text = "Button";

Okay, so I found out that I needed to add “UnityEngine.UI”.