Button Press and Hold, instead of Click?

I am creating an arcade-tank game and the longer you hold the space bar (if you're using a PC/Mac), the more force will apply. I want to port over to mobile devices, so how can I set it to the longer the press and hold the button to apply more force?

Here is my code for the PlayerAttack:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 using Shell;
 using UnityEngine.EventSystems;
 
 public class PlayerAttack : MonoBehaviour
 {
     public Rigidbody Shell;
     public Transform FireTransform;
     public Slider AimSlider;
     public float MinForce = 15f;
     public float MaxForce = 30f;
     //public float MaxForce = TankLevel.CurrentLevel * 6f;
     public float MaxCharge = 0.75f;
     public bool Fired;
     public Button BFire;
 
     string FireButton;
     float CurrentForce;
     float ChargeSpeed;
 
     void OnEnable ()
     {
         CurrentForce = MinForce;
         AimSlider.value = MinForce;
     }
 
     void Start ()
     {
         FireButton = "Fire";
 
         ChargeSpeed = (MaxForce - MinForce) / MaxCharge;
     }
 
     void Update ()
     {
         AimSlider.value = MinForce;
 
         if (CurrentForce >= MaxForce && !Fired) 
         {
             CurrentForce = MaxForce;
             Fire ();
         } 
         else if (Input.GetButtonDown (FireButton)) 
         {
             Fired = false;
             CurrentForce = MinForce;
         } 
         else if (Input.GetButton (FireButton) && !Fired) 
         {
             CurrentForce += ChargeSpeed * Time.deltaTime;
 
             AimSlider.value = CurrentForce;
         } 
         else if (Input.GetButtonUp (FireButton) && !Fired) 
         {
             Fire ();
         }
     }
 
     public void Fire ()
     {
         Fired = true;
 
         Rigidbody shellInstance = Instantiate (Shell, FireTransform.position, FireTransform.rotation) as Rigidbody;
 
         shellInstance.velocity = CurrentForce * FireTransform.forward;
 
         CurrentForce = MinForce;
     }
 }