How can overwrite in C#

<pre lang="c#">
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO.Ports;
using System.IO;



namespace PortDataReceived
{
class Program
{
static void Main(string[] args)
{
SerialPort mySerialPort = new SerialPort("COM1");
if (mySerialPort.IsOpen == true) mySerialPort.Close();
mySerialPort.BaudRate = 2400;
mySerialPort.Parity = Parity.None;
mySerialPort.StopBits = StopBits.One;
mySerialPort.DataBits = 8;
mySerialPort.Handshake = Handshake.None;
mySerialPort.Open();
Console.WriteLine("Press any key to continue...");
Console.WriteLine();
mySerialPort.ReadTimeout = 500;
string indata = mySerialPort.ReadExisting();
if (string.IsNullOrEmpty(indata))
{
FileStream fs = new FileStream(@"c:\arjun.txt",FileMode.Append,FileAccess.Write);
try
{

StreamWriter sw = new StreamWriter(fs);
sw.WriteLine("0");
sw.Close();
}
finally
{
fs.Close();
}

}
else
{
StringBuilder builder = new StringBuilder();
for (int i = 0; i < indata.Length; i += 8)
{

string section = indata.Substring(i, 8);

int ascii = 0;
try
{
ascii = Convert.ToInt32(section, 2);
}
catch
{
Console.WriteLine("Current Weight : " + section.ToString());
FileStream fs = new FileStream(@"c:\arjun.txt", FileMode.Append, FileAccess.Write);
try
{
StreamWriter sw = new StreamWriter(fs);

foreach (char c in section)
{
if (Char.IsLetterOrDigit(c))
{
builder.Append(c);
}
}

sw.WriteLine(builder.ToString());
sw.Close();
break;
}
finally
{
fs.Close();

}

}

}
}
// Console.WriteLine(builder.ToString());
Console.WriteLine(indata);
Console.WriteLine("Data Received:");
Console.Write(indata);
Console.ReadKey();
mySerialPort.Close();
}
}
}


</pre>


hello all the above is my code in this code i am read data from com port but problem is i was received 6 to 7 zero than received actual data from comport i was not under what was problem in my code pls let me know i was very thank full to all.i want solution on priority pls