How to transform a object to another when a object of tag "Hitter" hit it?

How to transform a object to another when a object of tag “Hitter” hit it?
Any comment and solution is appreciated here!

Depends on what you mean by transform I am assuming that you mean change the current model of the object? If so then you need to disable the model you are currently using and instantiate a new one. Something to the effect of:

currentModel.SetActiveRecursively(false);
newMode.SetActiveRecursively(true);

Pretty sure you could also do something like:

GameObject meshA;
GameObject meshB;

OnColliderEnter()
{
  player.SetActive(false);
  MeshFilter meshFilter = player.GetComponent<MeshFilter>();
  meshFilter.mesh = meshB.GetComponent<MeshFilter>().mesh;
  player.SetActive(true);
}

Or have all the models you want as children of the main object then use:

currentModel.SetActive(false);
newModel = player.transform.Find("ModelB").gameObject;
newModel.SetActive(true);
currentModel = newModel;