Help with C# - destroy at a certain position not working

Hello. Im very new to programing so still have a lot to learn. But have been learning through reading the documentation, youtube tutorials and these forums.

I have a simple game where stuff f.e. cubes randomly spawn and go twards the player. So I wanted to make a simple script to make the cubes despawn when they go behind the player (player position is Z = -100).

My Script:

 public class destroyUnderZvalue : MonoBehaviour
 {
     public float destroyPosition = -105.0f;
 
     void update()
     {
         if (transform.position.z <= destroyPosition)
         {
             Destroy (gameObject);
         }
     }
 }
 

Tho I dont know it its <= or >= since the value is lower (-105 is lower than -100)

But when I attatch it to the cubes it does nothing. Thank you in Advance.