hi people ive recently got into unity and wondering how to change the player speed?

using System.Collections;
using System.Collections.Generic;
using System.ComponentModel.Design.Serialization;
using UnityEditor;
using UnityEngine;

public class Player : MonoBehaviour
{
[SerializeField] private Transform groundCheckTransform = null;
[SerializeField] private LayerMask playerMask;
[SerializeField] private float horizontalSpeed = 4;

private bool jumpKeyWasPressed;
private float horizontalInput;
private Rigidbody rigidbodyComponent;
private int superJumpsRemaining;
private float speedBoost;
          

// Start is called before the first frame update
void Start()
{
    rigidbodyComponent = GetComponent<Rigidbody>();
}

// Update is called once per frame
void Update()
{
    //Check if space key is pressed down
    if (Input.GetKeyDown(KeyCode.Space))
    {
        jumpKeyWasPressed = true;
    }

    horizontalInput = Input.GetAxis("Horizontal");
}

//FixedUpdate is called once every physic update
private void FixedUpdate()
{
    rigidbodyComponent.velocity = new Vector3(horizontalInput * horizontalSpeed, rigidbodyComponent.velocity.y,0);

    if (Physics.OverlapSphere(groundCheckTransform.position, 0.1f, playerMask).Length == 0)
     {
        return;
     }

    if (jumpKeyWasPressed)
    {
        float jumpPower =5;
        if (superJumpsRemaining > 0)
        {
            jumpPower *= 2;
            superJumpsRemaining--;
        }
        rigidbodyComponent.AddForce(Vector3.up * jumpPower, ForceMode.VelocityChange);
        jumpKeyWasPressed = false;
    }
}


private void OnTriggerEnter(Collider other)
{
    if (other.gameObject.layer == 9)
    {
        Destroy(other.gameObject);
        superJumpsRemaining++;
    }
}

}