Unity 5 2D How to make a cursor move with keys?

Hello everybody,

I have a problem that may sound strange, but I still want to try to ask it. How do you make a cursor (aim cursor) and make it move not with the mouse, but with keys. I also have a animation so that when you aim to a certain point, the turret will aim that way (it follows the cursor) but I still can't figure it out. So I to aim up K to aim down J to aim left and L to aim right. I do not know how to make it so i am asking here because to me this looks like a mayor wall that i just can't breach. Here I have the script I use now (it moves shoots towards the cursor (mouse):

            using UnityEngine;
             using System.Collections;
             public class BulletPrototype1 : MonoBehaviour

        {

 public float maxSpeed = 25f;
 public GameObject Bullet;

 private Transform _myTransform;
 private Vector2 _lookDirection;

 private void Start()
 {
     if (!Bullet)
     {
         Debug.LogError("Bullet is not assigned to the script!");
     }

     _myTransform = transform;
 }

 private void Update()
 {
     /*
     mousePos - Position of mouse.
     screenPos2D - The position of Player on the screen.
     _lookDirection - Just the look direction... ;)
     */

     // Calculate 2d direction
     // The mouse pos
     var mousePos = new Vector2(Input.mousePosition.x, Input.mousePosition.y);

     // Player Camera must have MainCamera tag, 
     // if you can't do this - make a reference by variable (public Camera Camera), and replace 'Camera.main'.
     var screenPos = Camera.main.WorldToScreenPoint(_myTransform.position);
     var screenPos2D = new Vector2(screenPos.x, screenPos.y);

     // Calculate direction TARGET - POSITION
     _lookDirection = mousePos - screenPos2D;

     // Normalize the look dir.
     _lookDirection.Normalize();
 }

 private void FixedUpdate()
 {
     if (Input.GetButtonDown("Fire1"))
     {
         // Spawn the bullet
         var bullet = Instantiate(Bullet, _myTransform.position, _myTransform.rotation) as GameObject;

         if (bullet)
         {
             // Ignore collision
             Physics2D.IgnoreCollision(bullet.GetComponent<Collider2D>(), GetComponent<Collider2D>());

             // Get the Rigid.2D reference
             var rigid = bullet.GetComponent<Rigidbody2D>();

             // Add forect to the rigidbody (As impulse).
             rigid.AddForce(_lookDirection * maxSpeed, ForceMode2D.Impulse);

             // Destroy bullet after 5 sec.
             Destroy(bullet, 5.0f);
         }
         else
             Debug.LogError("Bullet not spawned!");
     }
 }

}

Can someone explain me how to do it, and if you can with an example script so that I can see what you mean? Because I need to understand the principle of the script.

Thank you in advance and have a nice day, Daniel.