How to change velocity of object which move by Rigidbody.

 private Rigidbody rb;
 
 void Start(){
    rb = GetComponent<Rigidbody>();
 }
 
 void Update(){
     if(Input.GetKeyDown(KeyCode.D){ // we speed up
         rb.velocity *= 1.1f;
    }else if(Input.GetKeyDown(KeyCode.A) // we slow down
         rb.velocity *= 0.9f;
     }
 }

If you want to go back to normal speed just do rb.velocity *= 1. Im also working on roller coaster and works fine.