Move a whole enemy object to a new point

In my code I am trying to check if the player dodges the obstacle and the obstacle goes to the x position of -13 (offscreen). If it does than it should reappear coming in with a random x and y coordinate. I get no errors it just doesn't do the code properly as I intended. Here is my code:

 using UnityEngine;
 using System.Collections;
 
 public class Obstacle : MonoBehaviour
 {
    public GameObject sphere;
    public Vector2 velocity = new Vector2(-4f, 0f);
    public float x_range=0f;
    public float y_range=0f;
 
    // Use this for initialization
    void Start()
    {
        rigidbody2D.velocity = velocity;
    }
 
    void update()
    {
        if (transform.position.x < -13f)
        {
            //random x
            x_range=Random.Range(15,30);
            //random y
            y_range=Random.Range(-12,7);
            sphere.transform.position=new Vector3(x_range,y_range,transform.position.z);
         }
     }
 }

Again not sure what the issue is. I am using Unity 2d, C#. I am a noob since this is only my 4th day with Unity and C# so go easy on me :)

Thanks in advance!