php laravel eloquent object can not be changed

I have a weird bug, I dont know why it will not work. So I am using Laravel php framework and getting from db a object. Since some decimal number looks like this : ‘.00’, I want to change it to this ‘0’, but it cannot be changed when I loop through it.
It looks like this:

 $stipendien = Stipendium::where('id','=',$request->input('id'))->get()->toArray();


        
        foreach($stipendien as $object => $test)
        {
            foreach($test as $key => $value)
            {
                if($test[$key] == ".00")
                {
                    $test[$key] = "0";
                }
            }
        }
return $stipendien;

Should not the ‘$stipendien’ be changed when I assign the value new, since it does go to the if statement and getting the correct key ?

array:2 [
  0 => array:17 [
    "id" => 1
    "Kosten_Kurs" => "22.00"
    "Kosten_Prüfung" => ".00"
    "Kosten_Unterlagen" => ".00"
    "Kosten_Lernurlaub" => ".00"
    "Kosten_Reise" => ".00"
    "Kosten_Sonstig" => ".00"

  ]
  1 => array:17 [
    "id" => 2
    "Kosten_Kurs" => "22.00"
    "Kosten_Prüfung" => "2.00"
    "Kosten_Unterlagen" => ".00"
    "Kosten_Lernurlaub" => ".00"
    "Kosten_Reise" => ".00"
    "Kosten_Sonstig" => ".00"
  ]
]

But when I return it, it did not change at all. What did I wrong here?