How do I change the raycasthit material back?

Please format your code by highlighting it and clicking the '101010' button, as it says in the user guide on the right. Makes it much easier to read, as well as not losing stuff inside angle brackets.

After the raycast, but before you set the obj value, check to see if it's different than the hit object. If it's different, reset the obj's material, then set obj to the new hit target. This also avoids it constantly trying to set the material of the object it's pointed at.

You also need to make sure obj is not null before changing its value in the else section or you'll get a Null Reference Exception right from the start if not pointing at anything.

 using UnityEngine;
 using System.Collections;
 
 public class TestScript : MonoBehaviour
 {
     public Vector3 vct;
     public Vector3 fwd;
     RaycastHit hit;
 
     GameObject obj;
     Renderer objRenderer;
     public Material newMtl;
     Material oldMtl;
 
     void Update()
     {
         if(Physics.Raycast (vct, fwd, out hit))
         {
             if(obj != hit.collider.gameObject)//If we're not pointing at the previous target
             {
                 if(obj != null)//If previous target is set, reset its material
                 {
                     objRenderer.material = oldMtl;
                 }
                 
                 obj = hit.collider.gameObject;//Store reference of target to a variable
                 objRenderer = obj.GetComponent<Renderer>();//Get targets Renderer
                 oldMtl = objRenderer.material;//Store targets current material
                 objRenderer.material = newMtl;//Set target to new material
             }
         }else{//If we're not pointing at anything
             if(obj != null)
             {
                 objRenderer.material = oldMtl;//Reset targets material
                 obj = null;//Clear reference
             }
         }
     }
 }

I also have it store the Renderer of the target to reduce GetComponent() usage, as it's a slow operation. Finally, I have it set oldMtl to the targets material, so targets can have different materials if you want.