I need help for delete old highscore

This script is game time. I want to save the maximum time playing, but instead each time you go back to play time adds up to high score. I need to delete the old time if you have exceeded the record. but what it does is to add two.I do not understand why. I'm using a text file to save my time. i guess this error. How i clear old score to put the new. thanks for the help. heres my script

using UnityEngine; using System.Collections;

public class TimeCounter : MonoBehaviour {

 public static TimeCounter time;

 public float time1;
 public float maxtime;
 public static bool stop = false;


 void Awake (){

     GameControl.control.time = Time.time;

 }

 void Update () {


     if(maxtime != 0){
         Time.timeScale = 1.0f;

     }
     if(stop == true){
         Time.timeScale = 0.0f;

     }
     if(stop == false){
         Time.timeScale = 1.0f;
     }



 }
 void OnLevelWasLoaded(int level) {
     if (level == 1)
         stop = false;
         Time.timeScale = 1.0f;
     if (level == 2)
         stop = true;
         Time.timeScale = 0.0f;

 }
 void ResetTimer() {

     stop = false;
 }

 void OnGUI () {

     maxtime = Time.timeSinceLevelLoad;
     GUI.Label(new Rect(100, 70, 100, 30), "Time: " + maxtime );
     
 }

}