Increase speed over time enemy

Hi there, i am trying to create a enemy who spawns and move towards y position with my knowledge i got this.I can move the object on Y position, but cant make it increase speed over time.

private Rigidbody2D rb2d;

public float speedIncrement = 10f;
public float maximumSpeed = 70f;
public float speed = 2;
public float scroolSpeed1 = 2;

void Start()
{      
    ////Get and store a reference to the Rigidbody2D attached to this GameObject.
    rb2d = GetComponent<Rigidbody2D>();
    ////Start the object moving.
    rb2d.velocity = new Vector2(0, 1) * scroolSpeed1;               
}

void FixedUpdate()
{
    scroolSpeed1 += speedIncrement;
    if (scroolSpeed1 <= maximumSpeed) scroolSpeed1 = maximumSpeed;        
}

rb2d.velocity = new Vector2(0, 1) * scroolSpeed1; this command line must be in FixedUpdate method after you increase speed

 void FixedUpdate()
 {
     scroolSpeed1 += speedIncrement;
     if (scroolSpeed1 <= maximumSpeed) scroolSpeed1 = maximumSpeed; 

rb2d.velocity = new Vector2(0, 1) * scroolSpeed1;        
 }