How to replace a line in a text file if string starts with?

Question

My Problem:

I am trying to replace a line in a text file with another line based on the condition of the string and a checkbox. For example, there are 2 checkbox, if 1 is checked, it should find string X and replace is with A

If checkbox 2 is checked, it finds string X and replaces it with B

My Research:

I have found that you cannot individually replace a specific line in a text file, but you must rewrite the file and exclude the line. This leaves me with a dilemma since i need that line to be replaced and not just removed. I have made efforts through research

My Efforts:

        DialogResult openFile = openFileDialog1.ShowDialog();
        if (openFile == DialogResult.OK)
        {
            string file = openFileDialog1.FileName;
            string content = File.ReadAllText(file);
            SaveFileDialog sfd = new SaveFileDialog();
            sfd.Filter = "Text File|*.txt";
            sfd.FileName = "New Text Doucment";
            sfd.Title = "Save As Text File";
            if (sfd.ShowDialog() == DialogResult.OK)
            {
                string path = sfd.FileName;
                StreamWriter bw = new StreamWriter(File.Create(path));
                bw.WriteLine(content);
                bw.Close();
                File.WriteAllLines(path, File.ReadAllLines(path).Select(x => string.Format("{0},", x)));
                string newContent = File.ReadAllText(path);
                newContent = newContent.Remove(newContent.LastIndexOf(","));
                File.WriteAllText(path, newContent);
                StreamReader reader = new StreamReader(path);
                string eachLine = reader.ReadLine();
                foreach (char value in newContent)
                {
                    if (eachLine.StartsWith("BRUSH"))
                    {
                        if (checkBox1.Checked == true)
                        {
                            var oldLines = File.ReadAllLines(path);
                            var newLines = oldLines.Where(line => !line.Contains("BRUSH"));
                            File.WriteAllLines(path, newLines);
                        }
                        else if (checkBox2.Checked == true)
                        {
                            var oldLines = File.ReadAllLines(path);
                            var newLines = oldLines.Where(line => !line.Contains("BRUSH"));
                            File.WriteAllLines(path, newLines);
                        }
                    }
                }
            }
        }

What i am doing :

• Grabbing an existing file,

• adding the contents to a new file,

• add a comma at the end of each line,

• removing the last comma at the end of the file.

  And now i need to replace the string in the file BRUSH with )), if checkbox 1 is checked, or replace BRUSH with string.empty is checkbox 2 is checked