Using Streamwriter to write to a specific place in a text file

I am trying to write to a specific position in a file. In the previous windows form I have selected a person to play with. In the file you will have three lines then a blank line then new player etc. I am a student and I cannot get this writing properly It is deleting everything in the text file. Can someone give me a few tips here because I cannot find any information on the internet to help me with writing to specific places in a text file. namespace Match_Picture {

public partial class Game : Form
{

    string Temp_Tag = "";
    int Number_of_Images = 0;
    Label Clicked_Picture = new Label();
    Label Temp_Pic = new Label();
    int Num_Correct = 0;
    int Timer = 0;
    int Complete_Time = 7000;
    int Time = 0;

    public Game()
    {
        InitializeComponent();


    }

    struct Score
    {
        public string Name;
        public int low;
        public int high;
    }

    static Score[] scores;



    private void BTN_StartGame_Click(object sender, EventArgs e)
    {

        lbl_1.Visible = true;
        lbl_2.Visible = true;
        lbl_3.Visible = true;
        lbl_4.Visible = true;
        lbl_5.Visible = true;
        lbl_6.Visible = true;
        lbl_7.Visible = true;
        lbl_8.Visible = true;
        lbl_9.Visible = true;
        lbl_10.Visible = true;
        lbl_11.Visible = true;
        lbl_12.Visible = true;
        lbl_13.Visible = true;
        lbl_14.Visible = true;
        lbl_15.Visible = true;
        lbl_16.Visible = true;

        Timer = 0;
        tmr_Time_Taken.Enabled = true;

        scores = new Score[4];

        StreamReader sr = new StreamReader(@"..\..\..\Files\Match.txt");


        for (int x = 0; x < 4; x++)
        {
            scores[x].Name = sr.ReadLine();
            scores[x].low = Convert.ToInt32(sr.ReadLine());
            scores[x].high = Convert.ToInt32(sr.ReadLine());
            sr.ReadLine();
        }
    }

    private void Picture_Click(object sender, EventArgs e)
    {
        Clicked_Picture = (Label)sender;

        Number_of_Images++;
        if (Number_of_Images < 3)
        {
            Clicked_Picture.Text = Clicked_Picture.Tag.ToString();

            if (Number_of_Images == 1)
            {
                Temp_Tag = Clicked_Picture.Tag.ToString();
                Temp_Pic = Clicked_Picture;
                Clicked_Picture.Enabled = false;
            }

            else
            {
                if (Temp_Tag != Clicked_Picture.Tag.ToString())
                {
                    tmr_Delay.Enabled = true;

                    Temp_Pic.Enabled = true;
                }
                else
                {
                    Num_Correct++;
                    lbl_Matches.Text = Num_Correct.ToString();
                    if (Num_Correct == 8)
                    {
                        if (Timer < Complete_Time)
                        {
                            Complete_Time = Timer;
                        }

                        tmr_Time_Taken.Enabled = false;


                        Time = Complete_Time;

                        //MessageBox.Show("Congratulations, all matches complete");
                        MessageBox.Show(string.Format("Congratulations, all matches complete in {0} seconds", Time), "Results", MessageBoxButtons.OK);



                        StreamWriter sw = new StreamWriter(@"..\..\..\Files\Match.txt");

                            for (int x = 0; x < 4; x++)
                            {
                                if (Complete_Time < scores[x].low)
                                {
                                    //string MyTime = Convert.ToString(Complete_Time);
                                    scores[x].low = Complete_Time; //sw.Write(Complete_Time);
                                    sw.Write(scores[x].low);

                                }
                                if (Complete_Time > scores[x].high)
                                {
                                    //string MyTime = Convert.ToString(Complete_Time);
                                    scores[x].high = Complete_Time; //sw.Write(Complete_Time);
                                    sw.Write(scores[x].high);
                                    //scores[x].high = Time;
                                    //scores[x].high = Convert.ToInt32(sr.ReadLine());
                                }

                            }

                    }
                    Clicked_Picture.Enabled = false;
                }
                Number_of_Images = 0;
            }
        }
    }