How To Update a Specific Line with StreamWriter?

use the string.Replace method if you know what you need to replace but you'll have to rewrite the entire text once you replaced your line.

also look: https://stackoverflow.com/questions/1971008/edit-a-specific-line-of-a-text-file-in-c-sharp

Bruce Afruz's answer is maybe a better alternative for you

 static void lineChanger(string newText, string fileName, int line_to_edit)
 {
      string[] arrLine = File.ReadAllLines(fileName);
      arrLine[line_to_edit - 1] = newText;
      File.WriteAllLines(fileName, arrLine);
 }