<?php
$hobby = $_POST['hobby'];
$query_update = "UPDATE user SET hobby='" .$hobby. "'
WHERE login = '" . $_SESSION['user_logged'] . "' AND haslo = '" . $_SESSION['user_password'] . "'";
<form action = "update_account.php" method="post">
<input type="checkbox" name="hobby" value="muzyka"> Muzyka
<input type="checkbox" name="hobby" value="Film" > Film
<input type="checkbox" name="hobby" value="Sport" > Sport
<input type="submit" name="submit" value="Uaktualnij">
<input type="button" value="Anuluj" onclick="history.go(-1);"> </form>
?>