Changing Update C#

Does any one know how to change Update to something else for example:
I have this code which lets my object move.... and i would like to change that code from (to make it stop)
THIS
transform.position += Vector3.up * Time.deltaTime*speed;
TO THIS
transform.position -= Vector3.up * Time.deltaTime*speed;
On Collision. This is my code:

Code (csharp):

1.  
2.  
3.     float speed = 5.0f;
4.    
5.  
6.     void Update() {
7.        
8.         transform.position += Vector3.up * Time.deltaTime*speed;
9.     }
10.     void OnCollisionEnter2D(Collision2D collision) {
11.         DestroyObject(collision.gameObject);
12.         transform.position -= Vector3.up * Time.deltaTime*speed;
13.  
14.  
15.     }
16.  
17.     public void SetSpeed(float modifier)
18.     {
19.         speed = 5.0f + modifier;
20.     }
21.  
22. }
23.  
24.  

 

I would create a variable, like direction, which is either 1 or -1, in CollisionEnter2D I would set it to -1, then I'd multiply the whole Vector3.up * Time.deltaTime * speed thing with it

 

not sure if this is what you're after. but it might get you on the right path

Code (CSharp):

1. float speed = 5.0f;
2. bool isFwd = true;
3.  
4.     void Update() {
5.        if(isFwd)
6.        {
7.         transform.position += Vector3.up * Time.deltaTime*speed;
8.         }else
9.         {
10.         transform.position -= Vector3.up * Time.deltaTime*speed;
11.         }
12.     }
13.    
14.     void OnCollisionEnter2D(Collision2D collision) {
15.         DestroyObject(collision.gameObject);
16.         isFwd = false;
17.     }
18.     public void SetSpeed(float modifier)
19.     {
20.         speed = 5.0f + modifier;
21.     }
22. }