Updating html form.

[killah]

I currently have my html payment form.

 

How can i make it so that a user can input the quantity and it update's the price & qty automaticly?

 

<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
	<input type="hidden" name="cmd" value="_xclick">
	<input type="hidden" name="business" value="'.$game['paypal'].'">
	<innut type="hidden" name="item_name" value="Deadly Killah|Donator Pack|1|'.$ir['userid'].'">
	<input type="hidden" name="amount" value="10.00">
	<input type="hidden" name="no_shipping" value="1">
	<input type="hidden" name="return" value="http://'.$_SERVER['HTTP_HOST'].'/'.$_SERVER['PHP_SELF'].'?donator=dppack&tp=paid">
	<input type="hidden" name="cancel_return" value="http://'.$_SERVER['HTTP_HOST'].'/'.$_SERVER['PHP_SELF'].'?donator=dppack&tp=cancel">
	<input type="hidden" name="notify_url" value="http://'.$_SERVER['HTTP_HOST'].'/ipnsystem.php">
	<input type="hidden" name="cn" value="'.$ir['userid'].'">
	<input type="hidden" name="currency_code" value="USD">
	<input type="hidden" name="tax" value="0">
	<input type="image" src="https://www.paypal.com/en_US/i/btn/x-click-but21.gif" border="0" name="submit" alt="Make Payment With Paypal Now!!! SECURE! FAST!">
</form>

 

I would like to update this <input type="hidden" name="amount" value="10.00"> with the new quantity * 10.00

 

And update this <innut type="hidden" name="item_name" value="Deadly Killah|Donator Pack|1|'.$user['userid'].'"> (1) with the new quantity

 

Any direction's and/or code's would be highly appreciated.