endless sidescroller porting to android

I have made an endless side scroller for pc I am trying to make it for android, mobile basically, I have a script that it will shoot a bullet but if it collides with an enemy, the enemy dies but if the enemy collides with the controller then I lose a life. The whole script works absolutely fine if you click the enter key on the key board (return key) here is the script if anyone can help me make this script work for android then thank you, any way thanks in advance.

 using UnityEngine;
 using System.Collections;
 
 public class HeroController : MonoBehaviour
 {
     public ProgressController ProgressController;
     public Transform HealthPickupAftermath;
     public Transform Bullet;
 
     void Start()
     {
     }
     
     void Update()
     {
         if (Input.GetKeyDown(KeyCode.Return))
         {
             Vector3 bulletPosition = transform.position;
             bulletPosition.y += 1.5f;
             bulletPosition.x += 1.0f;
             Transform newBullet = (Transform)Instantiate(Bullet, bulletPosition, Quaternion.identity);
             newBullet.rigidbody.AddForce(transform.forward * 100.0f);
             newBullet.parent = transform;
         }
     }
 
     void OnControllerColliderHit(ControllerColliderHit hit)
     {
         if (hit.gameObject.tag == "Enemy")
         {
             ProgressController.TakeDamage();
             MonsterController monsterController = hit.gameObject.GetComponent<MonsterController>();
             if (monsterController != null)
             {
                 monsterController.TouchedByHero();
             }
         }
         else if (hit.gameObject.tag == "Health")
         {
             ProgressController.Heal();            
             Instantiate(HealthPickupAftermath, hit.gameObject.transform.position, Quaternion.identity);
             Destroy(hit.gameObject);
         }
     }
 }

in this script there is a bit that says:

     void Update()
 if (Input.GetKeyDown(KeyCode.Return))

this part of the script makes it shoot when you press the enter/return key. I changed it to:

         void OnMouseDown ()
             if(true)

I then put the whole script on a GUI texture. It didn't work please some help me thanks in advanced