How can I make it so that the longer you hold down the jump button the higher the player jumps?

So I want the jumping in my game to be similar to games like Mario where if you tap the jump button the character does a small jump but if held down the character does a larger jump. How can I achieve this?

The code for my player movement currently:

 public float speedForce = 50f;
 public float jumpPower = 150f;

 public bool grounded;
 public bool canDoubleJump;
 
 private Rigidbody2D rb2d;
 private Animator anim;
 
 void Start ()
 {
     rb2d = gameObject.GetComponent<Rigidbody2D> ();
     anim = gameObject.GetComponent<Animator> ();
 }
 
 void Update ()
 {
     anim.SetBool ("Grounded", grounded);
     anim.SetFloat ("Speed", Mathf.Abs (rb2d.velocity.x));

     //Moving Left/Right
     if (Input.GetKey(KeyCode.RightArrow))
     {
         rb2d.velocity = new Vector2(speedForce, rb2d.velocity.y);
         transform.localScale = new Vector3(1, 1, 1);
     }
     else if (Input.GetKey(KeyCode.LeftArrow))
     {
         rb2d.velocity = new Vector2(-speedForce, rb2d.velocity.y);
         transform.localScale = new Vector3(-1, 1, 1);
     }
     else
         rb2d.velocity = new Vector2(0, rb2d.velocity.y);

     //Jumping
     if (Input.GetKeyDown (KeyCode.UpArrow))
     {
         if (grounded == true)
         {
             rb2d.AddForce(Vector2.up * jumpPower * 2f);
             canDoubleJump = true;
         }
         else
         {
             if(canDoubleJump == true)
             {
                 canDoubleJump = false;
                 rb2d.velocity = new Vector2(rb2d.velocity.x, 0);
                 rb2d.AddForce(Vector2.up * jumpPower * 2f);
             }
             
         }
     }
 }