Targeting system, can select but won't deselect...

Hi guys, I'm relatively new to Unity and c#. My question is regarding an MMO style targetting system similar to that of wow. I am able to select the target currently but cannot for the life of me workout how to deselect that target when another is selected. The results i have found are either old and the video responses are no longer available or unasnwered.

Here is my code so far.

using UnityEngine; using System.Collections;

 public class Targeting : MonoBehaviour {
     public bool isTarget = false;
     RaycastHit hit;
     Ray ray;
     public Transform target;
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         if (Input.GetButtonDown ("Fire1")) {
             ray = Camera.main.ScreenPointToRay (Input.mousePosition);
             if (Physics.Raycast (ray, out hit, 100)) {
                 if(hit.transform.tag == "Enemy"){
                 target = this.hit.transform;
                 ApplyTarget (target);
                 target.GetComponent<Targeting>().isTarget = true;
             } else {
                 target.renderer.material.color = Color.red;
                 target.GetComponent<Targeting>().isTarget = false;
                 }
             }
         }
     }
 
     void ApplyTarget(Transform theTarget){
             theTarget.renderer.material.color = Color.green;
     }
 }