How Can I make a prefab using navmesh alternate destination targets?

Hi guys,

I have a "Golem" prefab that is currently using a navmesh to move to the "end" destination. This works fine however, what I would like to do is: when a "cannon" appears in the scene, have the Golem prefab change its target to the cannon, and then change back again to "End" when the cannon is no longer in the scene.

Thanks in advance!

 public class GolemEnemy : MonoBehaviour {
  
      public GameObject Cannon;
      public GameObject End;
  
      public float DistanceFromCannon = 5.0f;
      public float Speed = 3f;
  
  
      // Use this for initialization
      void Start () {
  
          Cannon = GameObject.FindGameObjectWithTag("Cannon");
          if (!Cannon)
          {
              End = GameObject.FindGameObjectWithTag("End");
          }
  
      }
      
      // Update is called once per frame
      void Update () {
  
          if (Cannon)
          {
              GetComponent<NavMeshAgent>().destination = Cannon.transform.position;
          }
  
  
          else
          {
              GetComponent<NavMeshAgent>().destination = End.transform.position;
          }
      }
  }