Objects move down from top of screen,I want objects to randomly spawn at the top

use Random.Range to generate random numbers, you can use them for spawn locations and for moving too. you can make a random number from -1 to 1 so -1 makes it go left down, 0 straight down, 1 right down. or whatever you need.

So basically like...

 public GameObject CubeToSpawn;
 int RandomX;
 
 void Update()
 {
      GameObject spawnedCube = Instantiate(CubeToSpawn, new Vector3(RandomX, 6, 0), Quaternion.identity);
      StartCoroutine(Delay());
 }
 
 IEnumerator Delay()
 {
      yield return new WaitForSeconds(1);
 }