Convert WASD (or arrows) to touch input for ball maze game

Hello, I'm new to Unity. I've got a code that works fine to control a ball with WASD or arrow keys. but i want to be able to move the ball with guitexture arrows. For example, i will have one guiTexture arrow pointing up, and it ideally will apply positive vertical force like the W or upArrow key does. Below is my working code for keyboard controls, but i want it to work for guitexture. Please help me.

Oh, its being developed for android, if that makes any difference.

 using UnityEngine;
 
 using System.Collections;
 
 public class PlayerController : MonoBehaviour 
 {    
     public float speed;
     void FixedUpdate ()
     {
         float moveHorizontal = Input.GetAxis("Horizontal");
         float moveVertical = Input.GetAxis("Vertical");
         
         Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
         
         rigidbody.AddForce(movement * speed * Time.deltaTime);
     }