Edit in file code

 private void button1_Click(object sender, EventArgs e)
         {

             Sname = textBox1.Text;
             Spass = textBox2.Text;
             ArrayList all = new ArrayList();
                        ArrayList name = new ArrayList();
                        ArrayList pass = new ArrayList();
                        int k=0,i;
                        try
                         {
                            StreamReader z = new StreamReader("D:\\users.txt");

                       while (true)
                         {

                              string line = z.ReadLine();

                              if (line == null)
                               {  
                                  z.Close();
                                  break;
                               }
                             string[]words= line.Split(',');

                             all.Add(line);
                             name.Add(words[0]);
                             pass.Add(words[1]);

                           }
                              z.Close();
                             
                        
                          }
         
               catch (FileNotFoundException)
                              { MessageBox.Show("file not found"); }
 

             for ( i = 0; i < name.Count; i++)
                         {
                            if (name.Contains(label3.Text) && pass.Contains(label6.Text))
                               {
                                  k = pass.IndexOf(Spass);
                                  break;
                               }
                            if (k < all.Count)
                            {
                                all.RemoveAt(k);
                                all.Insert(k, Sname + "," + Spass);
                            }
                           
                            
                          
                            
                           
 try
 {
     StreamWriter m = new StreamWriter("D:\\users.txt");
    foreach (String s in all)
     { m.WriteLine(s);
     
     }
     m.Flush();
     m.Close();
 }

catch (FileNotFoundException)
 { MessageBox.Show("file not found"); }

 }
             this.Close();
         }