Dig on the terrain

Hi guys i know how to get the point on the terrain that my mouse has clicked but i wont to make that i can raise and lower the terrain(in the point i clicked) hear is the code to get the point on the terrain…

using UnityEngine;
using System.Collections;

public class DigSnow : MonoBehaviour {
public int rodentButton = 0;
public float rodentRange = 100f;
public string tagger = "terrain";
public GameObject prefab;
 
void Update(){
   if(Input.GetMouseButtonDown(rodentButton)){
      Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
      RaycastHit hit;
         if(Physics.Raycast(ray, out hit, rodentRange)){
            if(hit.transform.CompareTag(tagger)){
				Debug.Log(TerrainRenderFlags.heightmap);
               //il punto dove si è toccato il mondo è hit.point
               Debug.Log(hit.point);
				Instantiate(prefab,hit.point,Quaternion.identity);

            }
         }
    }
}
}

There are some examples,
http://forum.unity3d.com/threads/159026-Possible-to-make-player-editable-terrain-while-in-game?p=1088518&viewfull=1#post1088518