C# :: How To Update Text In WinForms Textbox

I am trying to create an application interface serial data reception using c #. I send the data from the serial adc microcontroller adc atmega 16 ... the data is 0 to 1023. In the textbox that I use as a data reception, data updates to the bottom if I use ReadLine coding.

examples:
1023
1023
1023
256
669

If I use ReadExisting, the data displayed by the textbox moves to the side. examples 102310231023256669

When the data changes from 0-1023 of serial communication, and clear the data will store in otomatiis in the textbox.

When there is reception of data with a value of 1023 and immediately there is a change to the value of 555 textbox value will not store the value of 1023, I want the value in 1023 immediately deleted automatically and change the value to 555 when a change in the value of the data ...

The following is the complete source code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
namespace serial {
public partial class Form1 : Form {

[Code] .....