how to change a line in a txt-document

Question

Hi there, I'm having a problem here. I have a .txt-file where one line contains "<message>", this is the line I wanna change. But I can't get this code to work, anyone that can help me?
I have this code here that is working for only replacing a string, but I don't know how to do it so it changes the whole line.

public void t()

        {

            string filename = @"F:\test\test.txt";

            StringBuilder result = new StringBuilder();

            if (System.IO.File.Exists(filename))

            {

                using (StreamReader streamReader = new StreamReader(filename))

                {

                    String line;

                    while ((line = streamReader.ReadLine()) != null)

                    {

                        string newLine = String.Concat(line, Environment.NewLine);

                        newLine = newLine.Replace("<message>", "HEJHEJ ");

                        result.Append(newLine);

                    }

                }

            }

            using (FileStream fileStream = new FileStream(filename, FileMode.Open, FileAccess.ReadWrite))

            {

                StreamWriter streamWriter = new StreamWriter(fileStream);

                streamWriter.Write(result);

                streamWriter.Close();

                fileStream.Close();

            }

        }

This code is changeing "<message>" to "HEJHEJ", but I want to whole line in the txt-document to change to "HEJHEJ", not just only "<message>"