Change to
'UPDATE `bands` SET `Stock` = (`Stock` - 1) WHERE `Band_id` = ' . $Band_id . ')';
If you're using PDO (and you probably should be), this is what you need:
$dsn = 'mysql:dbname=testdb;host=127.0.0.1';
$user = 'dbuser';
$password = 'dbpass';
$dbh = new PDO($dsn, $user, $password);
$dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$dbh->beginTransaction();
$dbh->exec('UPDATE `bands` SET `Stock` = (`Stock` - 1) WHERE `Band_id` = ' . $Band_id . ')');
$dbh->exec('INSERT INTO `orders` (`band_id`, `user_id`, `user_name`, `band_name`, `band_venue`) VALUES("'.$band_id.'", "'.$user_id.'", "'.$user_name.'", "'.$name.'", "'.$venue.'")');
$dbh->commit();
You could improve further by using prepared statements