public class NetworkManagerScript : MonoBehaviour {
  public int i = 100;  
  void Awake(){
    instance = this;
  }
  void Start () {
    Thread BackGrThrd = new Thread(new ThreadStart(NetWorkBackGrSupport.BGrPrnToConsole));
  }
  void Update () {
       Debug.Log ("I is "+i); // debug
  }
}

using UnityEngine;
using System.Collections;
using System.Threading;

public class NetWorkBackGrSupport{
   static int i = 0;
    public static void BGrPrnToConsole(){
     while(true){
      Thread.Sleep(1000);
      NetworkManagerScript.instance.i++;
    }  
  }  
}