Need a simple help here about moving up and down

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Myfile : MonoBehaviour
{
float speed = 50.0f;
float speed1 = -50.0f;
// Use this for initialization
void Start()
{

}

// Update is called once per frame
void Update()
{
if (Input.GetKey("d")) transform.Rotate(Vector3.up * speed * Time.deltaTime);
if (Input.GetKey("a")) transform.Rotate(Vector3.up * speed1 * Time.deltaTime);
}
}


So this what I made but still am confuse how to make it go up and down with Input key of W or S ?