Updating Bullet Force ONGUI

Hey guys, how do i update my Ongui so when the player presses A and S it either decreases or increases the power.

 using UnityEngine;
 using System.Collections;
 
 public class TurretControl : MonoBehaviour 
 {
     public NetworkPlayer owner;
     public float minForce, maxForce, forceIncrement;
     public float force;
     private int speed = 1;
     public float maxRotationZ;
     public float minRotationZ;
     
     void Awake()
     {
         enabled = false;
     }
     
     void Start()
     {
        minForce = 100;
        maxForce = 5000;
        forceIncrement = maxForce * 0.05f;
        force = (maxForce - minForce) / 2.0f;
 
         maxRotationZ = 0.5f;
         minRotationZ  = -0.1f;
    }
    void OnGUI()
    {
        GUI.Button(new Rect (200, 70, 200, 20), minForce + maxForce + forceIncrement);
    }
         
     void Update ()
     {
         if(Input.GetKey(KeyCode.UpArrow))
         {  
             //Debug.Log("z axis = " + transform.localRotation.z);
             if (transform.localRotation.z <= maxRotationZ)
             {
                 transform.Rotate(0, 0, Input.GetAxis("Vertical") * speed);
                 //Debug.Log("maxRotationZ:" + transform.rotation.z);
                 //Debug.Log("z axis = " + transform.localRotation.z);
             }
         }
         
         if(Input.GetKey(KeyCode.DownArrow))
         {
             //Debug.Log("z axis = " + transform.localRotation.z);
             if (transform.localRotation.z >= minRotationZ)
             {
                 transform.Rotate(0, 0, Input.GetAxis("Vertical") * speed);
                 //Debug.Log("minRotationZ:" + transform.rotation.z);
                 //Debug.Log("z axis = " + transform.localRotation.z);
                 
             }
         }
         
         if(Input.GetKey(KeyCode.A))
         {
             force += (forceIncrement * Time.deltaTime);
             
             if (force > maxForce) force = maxForce;
         }        
         else if(Input.GetKey(KeyCode.S))
         {
             force -= (forceIncrement * Time.deltaTime);
             
             if (force < minForce) force = minForce;
         }
     }
     
     public float getForce()
     {
         return force;
     }
 }