Changing a float so my character runs instead of walks

So I have a 2D platformer, and I was following a tutorial where they show you how to make the character move and walk. However I want to add a key press that increase movement speed so the character runs. I have tried to use my knowledge that I process to change the float within the code that is already used. However it does nothing. Can someone please explain to me how to make the variable increase with the key press then decrease when it's released?

Hear is my code. using.

Code (CSharp):

1. public class SHM_Script : MonoBehaviour
2.     {
3.         private Rigidbody2D SMRigidbody;
4.    
5.         private Animator SMAnimator;
6.    
7.         [SerializeField]
8.         private float movementSpeed;
9.    
10.         private bool facingRight;
11.    
12.         // Start is called before the first frame update
13.         void Start()
14.         {
15.             facingRight = true;
16.             SMRigidbody = GetComponent<Rigidbody2D>();
17.             SMAnimator = GetComponent<Animator>();
18.             movementSpeed = 10;
19.     }
20.    
21.         // Update is called once per frame
22.         void Update()
23.         {
24.             HandleInput();
25.         }
26.         void FixedUpdate()
27.         {
28.             float horizontal = Input.GetAxis("Horizontal");
29.    
30.             HandleMovement(horizontal);
31.    
32.             Flip(horizontal);
33.         }
34.    
35.         private void HandleInput()
36.         {
37.             if (Input.GetKeyDown(KeyCode.RightShift))
38.             {
39.                 movementSpeed = 20;
40.             }
41.             else
42.             {
43.                 movementSpeed = 10;
44.             }
45.         }
46.             private void HandleMovement(float horizontal)
47.         {
48.             SMRigidbody.velocity = new Vector2(horizontal * movementSpeed, SMRigidbody.velocity.y);
49.    
50.             SMAnimator.SetFloat("speed", Mathf.Abs(horizontal));
51.         }
52.    
53.         private void Flip(float horizontal)
54.         {
55.             if (horizontal > 0 && !facingRight || horizontal < 0 && facingRight)
56.             {
57.                 facingRight = !facingRight;
58.                 Vector3 theScale = transform.localScale;
59.                 theScale.x *= -1;
60.                 transform.localScale = theScale;
61.             }
62.         }
63.     }

The float in question is movement speed, and you will find the movement coding under the void handle movement. Thanks in advance.

 

It works!! Thank you so much. I will read that article, so thanks for that too.