Modify the content of the text in a GUi bottom

By GUI bottom do you mean Button? Because the button text is just another Text component so it's totally ok. Just find the text component and change it. public GameObject button;

 void Start()
 {
     button.transform.Find("Text").GetComponent<Text>().text = "whatever you want";
 }

Note that if you want to use UI you need to use its namespace: using UnityEngine.UI;