เมื่อเปลี่ยนจำนวนสินค้า แล้วให้อัพเดทราคารวมอัตโนมัติ โดยไม่ต้องกดอัพเดท ทำยังไงครับ

<form id="form1" name="form1" method="post" action="update.php" >
   <table width="80%" border="1" align="center" cellpadding="0" cellspacing="0">
                  <tr>
                    <td width="8%"><div align="center"><strong>รหัสเมนู</strong></div></td>
                    <td  width="18%"><strong>ชื่อเมนู</strong></td>
                    <td  width="10%"><div align="center"><strong>ประเภท</strong></div></td>
                    <td  width="10%"><div align="center"><strong>รูปภาพ</strong></div></td> 
                    <td  width="15%"><div align="center"><strong>ราคาต่อหน่วย(บาท)</strong></div></td>
                    <td  width="11%"><div align="center"><strong>หน่วยนับ</strong></div></td>
                    <td  width="8%"><div align="center"><strong>จำนวน</strong></div></td>
                    <td  width="15%"><div align="right"><strong>รวมราคา (บาท)</strong></div></td>
                    <td  width="5%">&nbsp;</td>
                  </tr>

<?php
 					$Total = 0;
  					$SumTotal = 0;
					

  					 for($i=0;$i<=(int)$_SESSION["intLine"];$i++)
  					{
	 					 if(isset($_SESSION["strMenuID"][$i]) != "")
	 					 {
							$queryuser = "SELECT * FROM menu WHERE MenuID = '".$_SESSION["strMenuID"][$i]."' ";
							$rsuser = mysql_query($queryuser)  or die(mysql_error());
							$rowuser = mysql_fetch_array($rsuser);
							
							if ($rowuser){
							
							$Total = $_SESSION["strAmount"][$i] * $rowuser["MenuPrice"];
							$SumTotal = $SumTotal + $Total;
?>
                     
                     <tr>
                       <td><div align="center"><?php echo $_SESSION["strMenuID"][$i];?>
                       <input type="hidden" name="txtMenuID <?php echo $i;?>" value="<?php echo $_SESSION["strMenuID"][$i];?>"></div></td>
                     	<td><div align="left"><?php echo $rowuser["MenuName"];?></div></td>
                        <td><div align="center">
					   		<?php 
									if( $rowuser["MenuType"] == 1)
										echo "อาหาร";
									else if ( $rowuser["MenuType"] == 2)
										echo "เครื่องดื่ม";
									else if ( $rowuser["MenuType"] == 3)
										echo "ไข่";
							?></div></td>

                     	<td><div align="center"><img src="img_menu/<?php echo $rowuser["MenuImage"];?>" width="50" height="50"></div></td> 
                        <td><div align="center"><?php echo $rowuser["MenuPrice"];?></div></td>     
                        <td><div align="center">
					   		<?php 
									if( $rowuser["MenuUnit"] == 1)
										echo "จาน";
									else if ( $rowuser["MenuUnit"] == 2)
										echo "แก้ว";
									else if ( $rowuser["MenuUnit"] == 3)
										echo "ฟอง";
							?></div></td>
                        <td><div align="center"><select name="qty<?php echo $i;?>" />
				<?php 
			
				for($qty=1;$qty<=20;$qty++)
			  {
					$sel = "";
					if($_SESSION["strAmount"][$i] == $qty)
				  {
						$sel = "selected";
				  }
			  ?>
				<option value="<?php echo $qty;?>" <?php echo $sel;?> ><?php echo $qty;?></option>
				<?php
			  }
			  ?>
			</select></td>
                        <td><div align="right"><?php echo number_format($Total,0);?></div></td>
                     	<td><div align="center"><a href="delete.php?Line=<?php echo $i;?>" onclick="return confirm('ยืนยันการลบเมนูที่เลือก !!!')">ลบ</a></div></td></tr>
               	 	<?php
	  					}
  					}
					}
					?>
                </table>
    
<table width="80%" border="0" align="center">
<tr>
<td width="8%"></td>
<td  width="20%"></td>
<td width="8%"></td>
<td width="11%"></td>
<td width="15%">              <?php
			if($SumTotal > 0)
			{
			?>
              <input type="submit" name="button" id="button" value="คำนวณใหม่" />
              <?php
			}
			?></td>
<td width="19%"><div align="right"><strong>รวมทั้งหมด</strong></div></td>
<td width="14%"><div align="right"><strong><?php echo number_format($SumTotal,0);?></strong>
</div></td>
<td width="5%"></td>
</tr>
</table>