My script its udpating all fields when i just edit one : my problem its im trying to develop a backend where i need to update but my problem its , when I update one field my script update all fields and all data of my mysqli database.
My code for now its that : 


    <html>
    <body>
    <?php
      
      ini_set('display_errors', 1);
       error_reporting(~0);
    
       $serverName = "localhost";
       $userName = "root";
       $userPassword = "";
       $dbName = "hotel_vaniet";
    
       $strCustomerID = null;
    
       if(isset($_GET["cod"]))
       {
    	   $cod = $_GET["cod"];
       }
      
      
       $serverName = "localhost";
       $userName = "root";
       $userPassword = "";
       $dbName = "hotel_vaniet";
    
       $conn = mysqli_connect($serverName,$userName,$userPassword,$dbName);
    
       $sql = "SELECT * FROM quartos WHERE cod=$cod";
    
       $query = mysqli_query($conn,$sql);
    
       $result=mysqli_fetch_array($query,MYSQLI_ASSOC);
    
    ?>
    <div id="main">
    <form action="editar_quartos_final.php" name="frmAdd" method="post">
    <br><h1>Página de Edição</h1>
    <br><hr/>
    <div id="login2">
    <table width="284" border="1">
      <tr>
        <th width="120">Tipo</th>
        <td width="238"><input type="text" name="tipo" size="50" value="<?php echo $result["tipo"];?>"></td>
        </tr>
      <tr>
        <th width="120">Capacidade</th>
        <td><input type="text" name="capacidade" size="50" value="<?php echo $result["capacidade"];?>"></td>
        </tr>
      <tr>
        <th width="120">Preço p/ Noite</th>
        <td><input type="text" name="preco" size="50" value="<?php echo $result["preco"];?>"></td>
        </tr>
      <tr>
        <th width="120">Reservado</th>
        <td><input type="text" name="reservado" size="50" value="<?php echo $result["reservado"];?>"></td>
        </tr>
      </table>
      <br><input id="submitbuttoneditar" type="submit" value=" Editar " name="submit"/><br />
      </div>
    </form>
    
    <?php
    mysqli_close($conn);
    ?>
    </body>
    </html>

This the first page ,this page send me to another where makes all changes. The second page : 

    <html>
    <head>
    <title>Página de Edição do Cliente</title>
    </head>
    <body>
    <?php
    $servername = "localhost";
    $username = "root";
    $password = "";
    $dbname = "hotel_vaniet";
    
    // Create connection
    $conn = new mysqli($servername, $username, $password, $dbname);
    // Check connection
    if ($conn->connect_error) {
        die("Connection failed: " . $conn->connect_error);
    } 
    
    $sql = "UPDATE quartos SET 
    			tipo = '".$_POST["tipo"]."' ,
    			capacidade = '".$_POST["capacidade"]."' ,
    			preco = '".$_POST["preco"]."' ,
    			reservado = '".$_POST["reservado"]."'
    			WHERE cod=cod";
    
    if ($conn->query($sql) === TRUE) {
        echo "Dados actualizados com sucesso!";
    	header("Location: quartos.php");
    } else {
        echo "Erro na edição dos dados! " . $conn->error;
    	header("Location: quartos.php");
    }
    
    $conn->close();
    ?>
    </body>
    </html> 

Thanks for your help !