function ajaxFunction(str,id) {
jQuery.ajax({
                url: '<?php echo admin_url('admin-ajax.php'); ?>',
                type: 'post',
                data: {
                    'action' : 'my_action',
                    'payment_selected': payment_selected,
                    'id'    : id
                },
                success: function (response) {
                    if(response=='success'){
                    jQuery("#status").html(response);
                    }
                }
});
}

add_action( 'wp_ajax_nopriv_my_action', 'my_action' );
add_action( 'wp_ajax_my_action', 'my_action' );

function my_action() {
 global $wpdb;
 $id_selected = $_POST['payment_selected'];
 $id = $_POST['id'];
 $table_name_payment = $wpdb->prefix . "online_booking_system_model";
 $result_pay =  $wpdb->query($wpdb->prepare("UPDATE $table_name_payment SET payment_status = $id_selected WHERE id=$id"));
 echo "success";
}
?>

<?php foreach ($rows as $row) { ?>
 <table>
 <tr><td><?php echo $row->payment_status; ?></td>
 <select  name='payment_select' id="payment_select" onchange="ajaxFunction(this.value,<?php echo $row->id ?>)">
        <option value="Payment Due">Payment Due</option>
        <option value="Payment completed">Payment Completed</option>
 </select>
 <?php } ?>
 </td></tr>
 <table>
 <div id="status"></div>