change gravity after time

Hi everyone, I have spawners, which create very 5 sceonds some objects. Now I want, that they change every 5 seconds the gravity too, but how can I change it?
Sorry for bad english, thank you!

var cube : GameObject;
var spawnPosition : GameObject;
var timer = 0.0;

function SpawnCube() {

    var tempCubeSpawner = Instantiate(cube, spawnPosition.transform.position, Quaternion.identity);

}

function Update() {

    timer += Time.deltaTime;
    if(timer > 5) {
    var randomNumber = Random.Range(0.0f,1.0f);
    
    if(randomNumber < 0.5f) {
      SpawnCube();
      timer = 0.0;
// Here change the gravity
    }
    else {
      timer = 0.0;
    }

    }

}