Script PHP update data tidak berhasil

Dear All,

Mohon pencerahannya terkait dengan script php saya, problemnya : tidak dapat mengupdate data yang sudah di edit.

1. Form editguru.php :

<?php require_once 'header.php'; require_once 'aside.php';?>
	<main>
		<h4>Home / Daftar Guru</h4>
		<hr/>
		<br/>
		<form action="updateguru.php" method="POST">
		<?php
		$link 		 = mysqli_connect('localhost','root','','db_sekolahkoding');
		$id	  		 = $_GET['edit'];

		$edit   	 = mysqli_query($link, "SELECT * FROM tblguru where id_guru='$id'");
		$showguru    = mysqli_fetch_array ($edit);

		// cek koneksi
		// if (!$link){
		// die ('tidak konek'. mysqli_connect_error());
		// }else
		// echo 'berhasil';

		?>
		<br>
		<div>
			<button name="update">Update</button>
			<button name="cancel">Batal</button>
		</div>
		<br>
		<div>
				<table>
					<caption style="text-align:center;"><h4>REGISTRASI DATA GURU<h4></caption>
					<input type="hidden" id="id_guru" name="id_guru" value="<?php echo $showguru['id_guru']; ?>">
				<tr>
					<td><label for="nip">Nip</label></td>
					<td><input type="text" id="nip" name="nip" value="<?php echo $showguru['Nip']; ?>"></td>
				</tr>
				<tr>
					<td><label for="namaguru">Nama</label></td>
				    <td><input type="text" id="namaguru" name="namaguru" value="<?php echo $showguru['Nama']; ?>"></td>
				</tr>
				<tr>
					<td><label for="bidangstudi">Bidang Studi</label></td>
					<td>
						<Select id="bidangstudi" name="bidangstudi">
						<?php
							$showstudi = mysqli_query ($link, "SELECT * FROM tbldibangstudi ORDER BY Bidangstudi ASC");
							while ($rowstudi  = mysqli_fetch_array($showstudi)){
								if ($showguru[id_bidangstudi] == $rowstudi[id_bidangstudi]){
									echo "<option value=\"$rowstudi[id_bidangstudi]\" selected>$rowstudi[Bidangstudi]</option>";
								}else{
									echo "<option value=\"$rowstudi[id_bidangstudi]\">$rowstudi[Bidangstudi]</option>";
								}
							}
						?>
						</Select>
					</td>
				</tr>
				<tr>
					<td><label for="nohpguru">No. Handphone</label></td>
				    <td><input type="text" id="nohpguru" name="nohpguru" value="<?php echo $showguru['Nohandphoneguru']; ?>"></td>
				</tr>
				<tr>
					<td><label for="alamatguru">Alamat</label></td>
					<td><input type="text" id="alamatguru" name="alamatguru" value="<?php echo $showguru['Alamat']; ?>"></td>
				</tr>
			    </table>
			    <br>
			</form>
		</div>
	</main>
<?php require_once 'footer.php'; ?>

2. Script updateguru.php :

<?php
$link 		 = mysqli_connect('localhost','root','','db_sekolahkoding');

$id_guru	    = $_POST['id_guru'];
//$id_bidangstudi = $_POST['id_bidangstudi'];
$nip		    = $_POST['nip'];
$namaguru	 	= $_POST['namaguru'];
$bidangstudi 	= $_POST['bidangstudi'];
$nohpguru	 	= $_POST['nohpguru'];
$alamatguru	 	= $_POST['alamatguru'];

$update		 = "UPDATE tblguru SET (Nip='$nip', Nama='$namaguru', Bidangstudi='$bidangstudi',
               Nohandphoneguru='$nohpguru', Alamat='$alamatguru' WHERE id_guru='$id_guru')";

if (isset($_POST['update'])) {
	 if (mysqli_query($link,$update)) {
	 	echo "<script language=\"javascript\">
		         alert (\"Data Berhasil Diupdate !!\")
		         document.location=\"daftarguru.php\";
		       </script>";
	 }else
	    echo "<script language=\"javascript\">
		         alert (\"Gagal Update Data\")
		         document.location=\"daftarguru.php\";
		      </script>";
}

 mysqli_close($link);
?>

Terima Kasih atas bantuan dan pencerahannya