$json = '{ 
  "cols": [ 
        {"id":"","label":"Topping","pattern":"","type":"string"}, 
        {"id":"","label":"Slices","pattern":"","type":"number"} 
      ], 
  "rows": [ 
        {"c":[{"v":"Mushrooms","f":null},{"v":3,"f":null}]}, 
        {"c":[{"v":"Onions","f":null},{"v":1,"f":null}]}, 
        {"c":[{"v":"Olives","f":null},{"v":1,"f":null}]}, 
        {"c":[{"v":"Zucchini","f":null},{"v":1,"f":null}]}, 
        {"c":[{"v":"Pepperoni","f":null},{"v":2,"f":null}]} 
      ] 
} ';
$arr = json_decode($json,true);
foreach ($arr['rows'] as &$row){
$yenisayi = 0;
$row['c'][1]['v'] = $yenisayi;

}
$sonjson = json_encode($arr);
echo $sonjson;