How can i upgrade the fire rate and damage in my upgrade menu

Hi I am new to unity and I know very little about coding and I don't know how can I upgrade the fire rate and the damage from my upgrade menu this is a game from brackeys 2d platformer course a link to it:https://www.youtube.com/watch?list=PLPV2KyIb3jR42oVBU6K2DIL6Y22Ry9J1c&v=UbPiCgCkHTE

a screenshot of the upgrade menu:https://prnt.sc/s5jdqt

a screenshot of the game:https://prnt.sc/s5je46

any help would be appreciated thanks a lot in advance :) Statusindicator script:

 using UnityEngine;
 using UnityEngine.UI;
 public class StatusIndicator : MonoBehaviour {
 [SerializeField]
 private RectTransform healthBarRect;
 [SerializeField]
 private Text healthText;
 void Start()
 {
 if (healthBarRect == null)
 {
 Debug.LogError("STATUS INDICATOR: No health bar object referenced!");
 }
 if (healthText == null)
 {
 Debug.LogError("STATUS INDICATOR: No health text object referenced!");
 }
 }
 public void SetHealth(int _cur, int _max)
 {
 float _value = (float)_cur / _max;
 healthBarRect.localScale = new Vector3(_value, healthBarRect.localScale.y, healthBarRect.localScale.z);
 healthText.text = _cur + "/" + _max + " HP";
 }
 }

Weapon script:

 using UnityEngine;
 using System.Collections;
 public class Weapon : MonoBehaviour
 {
 public float fireRate = 0;
 public int Damage = 10;
 public LayerMask whatToHit;
 public Transform BulletTrailPrefab;
 public Transform HitPrefab;
 public Transform MuzzleFlashPrefab;
 float timeToSpawnEffect = 0;
 public float effectSpawnRate = 10;
 // Handle camera shaking
 public float camShakeAmt = 0.05f;
 public float camShakeLength = 0.1f;
 CameraShake camShake;
 public string weaponShootSound = "DefaultShot";
 float timeToFire = 0;
 Transform firePoint;
 // Caching
 AudioManager audioManager;
 // Use this for initialization
 void Awake()
 {
 firePoint = transform.Find("FirePoint");
 if (firePoint == null)
 {
 Debug.LogError("No firePoint? WHAT?!");
 }
 }
 void Start()
 {
 camShake = GameMaster.gm.GetComponent<CameraShake>();
 if (camShake == null)
 Debug.LogError("No CameraShake script found on GM object.");
 audioManager = AudioManager.instance;
 if (audioManager == null)
 {
 Debug.LogError("FREAK OUT! No audiomanager found in scene.");
 }
 }
 // Update is called once per frame
 void Update()
 {
 if (fireRate == 0)
 {
 if (Input.GetButtonDown("Fire1"))
 {
 Shoot();
 }
 }
 else
 {
 if (Input.GetButton("Fire1") && Time.time > timeToFire)
 {
 timeToFire = Time.time + 1 / fireRate;
 Shoot();
 }
 }
 }
 void Shoot()
 {
 Vector2 mousePosition = new Vector2(Camera.main.ScreenToWorldPoint(Input.mousePosition).x, Camera.main.ScreenToWorldPoint(Input.mousePosition).y);
 Vector2 firePointPosition = new Vector2(firePoint.position.x, firePoint.position.y);
 RaycastHit2D hit = Physics2D.Raycast(firePointPosition, mousePosition - firePointPosition, 100, whatToHit);
 Debug.DrawLine(firePointPosition, (mousePosition - firePointPosition) * 100, Color.cyan);
 if (hit.collider != null)
 {
 Debug.DrawLine(firePointPosition, hit.point, Color.red);
 Enemy enemy = hit.collider.GetComponent<Enemy>();
 if (enemy != null)
 {
 enemy.DamageEnemy(Damage);
 //Debug.Log ("We hit " + hit.collider.name + " and did " + Damage + " damage.");
 }
 }
 if (Time.time >= timeToSpawnEffect)
 {
 Vector3 hitPos;
 Vector3 hitNormal;
 if (hit.collider == null)
 {
 hitPos = (mousePosition - firePointPosition) * 30;
 hitNormal = new Vector3(9999, 9999, 9999);
 }
 else
 {
 hitPos = hit.point;
 hitNormal = hit.normal;
 }
 Effect(hitPos, hitNormal);
 timeToSpawnEffect = Time.time + 1 / effectSpawnRate;
 }
 }
 void Effect(Vector3 hitPos, Vector3 hitNormal)
 {
 Transform trail = Instantiate(BulletTrailPrefab, firePoint.position, firePoint.rotation) as Transform;
 LineRenderer lr = trail.GetComponent<LineRenderer>();
 if (lr != null)
 {
 lr.SetPosition(0, firePoint.position);
 lr.SetPosition(1, hitPos);
 }
 Destroy(trail.gameObject, 0.04f);
 if (hitNormal != new Vector3(9999, 9999, 9999))
 {
 Transform hitParticle = Instantiate(HitPrefab, hitPos, Quaternion.FromToRotation(Vector3.right, hitNormal)) as Transform;
 Destroy(hitParticle.gameObject, 1f);
 }
 Transform clone = Instantiate(MuzzleFlashPrefab, firePoint.position, firePoint.rotation) as Transform;
 clone.parent = firePoint;
 float size = Random.Range(0.6f, 0.9f);
 clone.localScale = new Vector3(size, size, size);
 Destroy(clone.gameObject, 0.02f);
 //Shake the camera
 camShake.Shake(camShakeAmt, camShakeLength);
 //Play shoot sound
 audioManager.PlaySound(weaponShootSound);
 }
 }

Upgrade menu script:

 using UnityEngine;
 using UnityEngine.UI;
 public class UpgradeMenu : MonoBehaviour
 {
 [SerializeField]
 private Text healthText;
 [SerializeField]
 private Text speedText;
 [SerializeField]
 private float healthMultiplier = 1.3f;
 [SerializeField]
 private float movementSpeedMultiplier = 1.3f;
 [SerializeField]
 private int upgradeCost = 50;
 private PlayerStats stats;
 void OnEnable()
 {
 stats = PlayerStats.instance;
 UpdateValues();
 }
 void UpdateValues()
 {
 healthText.text = "HEALTH: " + stats.maxHealth.ToString();
 speedText.text = "SPEED: " + stats.movementSpeed.ToString();
 }
 public void UpgradeHealth()
 {
 if (GameMaster.Money < upgradeCost)
 {
 AudioManager.instance.PlaySound("NoMoney");
 return;
 }
 stats.maxHealth = (int)(stats.maxHealth * healthMultiplier);
 GameMaster.Money -= upgradeCost;
 AudioManager.instance.PlaySound("Money");
 UpdateValues();
 }
 public void UpgradeSpeed()
 {
 if (GameMaster.Money < upgradeCost)
 {
 AudioManager.instance.PlaySound("NoMoney");
 return;
 }
 stats.movementSpeed = Mathf.Round(stats.movementSpeed * movementSpeedMultiplier);
 GameMaster.Money -= upgradeCost;
 AudioManager.instance.PlaySound("Money");
 UpdateValues();
 }
 }

any help would be appreciated a lot ;)