How to set text when i want?

So i have a highscore text that i want to change when the player loses (later i wanna add live score track aswell) But i cant seem to figure out how to do it, this is the but in the code that are meant to change the highscore text

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class groundController : MonoBehaviour {
 
     
     public GameObject spawner;
     public GameObject greenBall;
     public GameObject redBall;
     public GameObject yellowBall;
     public Text highScoreText;
 
     void Start ()
     {
         
     }
     
     
     void Update ()
     {
         greenBall = GameObject.FindGameObjectWithTag("greenBall");
         redBall = GameObject.FindGameObjectWithTag("redBall");
         yellowBall = GameObject.FindGameObjectWithTag("yellowBall");
         Debug.Log(yellowBall);
         
     }
 
     void OnTriggerEnter2D(Collider2D other)
     {
         if (other.gameObject.tag == "greenBall")
         {
             GameOver();
         }
         else if (other.gameObject.tag == "yellowBall")
         {
             Destroy(other.gameObject);
         }
         else if(other.gameObject.tag == "redBall")
         {
             Destroy(other.gameObject);
         }
     }
 
     public void GameOver()
     {
         spawner.GetComponent<gameController>().playable = false;
         Destroy(greenBall);
         Destroy(redBall);
         Destroy(yellowBall);
         if (spawner.GetComponent<gameController>().Score > spawner.GetComponent<gameController>().highScore)
         {
             spawner.GetComponent<gameController>().highScore = spawner.GetComponent<gameController>().Score;
             highScoreText = "Highscore: " + spawner.GetComponent<gameController>().highScore;
         }
         else if(spawner.GetComponent<gameController>().Score == spawner.GetComponent<gameController>().highScore)
         {
 
         }
         else if(spawner.GetComponent<gameController>().Score < spawner.GetComponent<gameController>().highScore)
         {
             
         }
        
     }
 
     void ClearBalls()
     {
         if(spawner.GetComponent<gameController>().playable == false)
         {
             Destroy(greenBall);
             Destroy(redBall);
             Destroy(yellowBall);
         }
     }
 
 }

I KNOW I KNOW it is gore to a someone that know what they are doing but it works for me.

But i am getting the error "Cannot implicitly convert type 'string' to 'Unity.Engine.UI.Text' " (Line 54)

Any help here ?