Adding Value to Already Saved Values

I have this set up so that it saves the Coins Collected value and the loads it in another scene. Ill attach the two scripts below...What i want to do now is so the ObjectDestroy class will save its value as Save_Coins_Collected but when it saves the value, I want to add this to the previously saved value which would be loaded in the Upgrades_Script as TotalCoinsCollected. This will then be so the player collects coins and if they play again or close game and then play later, it will add the coins to there previous amount of coins. The coins will build up and allow them to buy things in game.

ObjectDestroy.cs

 void OnTriggerEnter(Collider other) //When a named trigger is entered...do something
     {
         
         if (other.tag == "ObjectDestroy") 
         {
             Debug.Log ("HITTTTTTTTT"); //display in console if object hits the ObjectDestroy tag
             Destroy (gameObject); // destroy object this script is attached to
         }
 
 
         if (other.tag == "Player")
         {
             Lives_2.Lives_Left --;
             Time.timeScale = 0; // stop the game if player tag is hit
 
             CoinsCollected.Coins_Collected = PlayerPrefs.GetInt ("Coins");
 
             BinaryFormatter bf = new BinaryFormatter();
             FileStream file = File.Create (Application.persistentDataPath + "/playerInfo.dat");
             
             PlayerData data = new PlayerData();
             data.Save_Coins_Collected = CoinsCollected.Coins_Collected;
             
             bf.Serialize (file, data);
             file.Close();
 
             Application.LoadLevel ("Lose_Screen"); //Load Lose_Screen scene
         }
     }
 
 }
 
 [Serializable]
 class PlayerData
 {
     public int Save_Coins_Collected;
 }
 

Upgrades_Script.cs

 {
 
     public static int TotalCoinsCollected;
 
     Text text;
     
     void Awake()
     {
         text = GetComponent<Text> ();
     }
 
     // Use this for initialization
     void Start () 
     {
 
         if (File.Exists (Application.persistentDataPath + "/playerInfo.dat")) 
         {
             BinaryFormatter bf = new BinaryFormatter();
             FileStream file = File.Open(Application.persistentDataPath + "/playerInfo.dat", FileMode.Open);
             PlayerData data = (PlayerData)bf.Deserialize(file);
             file.Close();
             
             TotalCoinsCollected = data.Save_Coins_Collected;
         }
 
         text.text = "Total Coins Collected: " + TotalCoinsCollected;
     
     }
 
 }

Any Help Appreciated.