Hmmm. Your a little vague here. Well actually your a ton vague and your providing only the minimal code.
First of this is PHP code, your asking for help in a JavaScript form.
Second off the second code box is not PHP or JavaScript, it is HTML
That being said if your trying to subtract a value from the variable $monto then just subtract it like this:
<?php
$monto=$monto-1000;
?>
if your trying to display the variable in the second code box change that line to this:
<input type="text" name="total" id="total" value="<?php echo $monto; ?>">