// If you are trying to chnage the text on a TextMeshPro:
public TextMeshPro TextMeshProObject;
//you shouldnt need to get component the editor should take care of this for you when
//you drop it since you have the object set to TextMeshPro and not just GameObject
TextMeshProObject = GetComponent<TextMeshPro>();
TextMeshProObject.text = "Hello";
// If you are trying to change the text from a gameobject:
TextMeshProUGUI TextMeshProLable = YourGameObject.GetComponent<TextMeshProUGUI>();
TextMeshProLable.text = "Your Text"