Update ASP.Net GridView cell value based on TextBox using C#

Change gridview value from Absent to Present if gridview value match outside textbox value 

I WANT TO CHANGE THE ATTENDANCE STATUS PRESENT INSIDE GRIDVIEW WHEN THE STUDENT ADMISSION NO IS ENETERED IN THE TEXTBOX OUTSIDE THE GRIDVIEW. HOW TO DO IT

        <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" Class="table table-striped table-bordered table-hover" OnRowDataBound = "OnRowDataBound">  
            <Columns>      
                <asp:TemplateField HeaderText="Admission No">  
                    <ItemTemplate>  
                        <asp:Label ID="lbl_AdNo" runat="server" Text='<%#Eval("AdmissionNo") %>'></asp:Label>  
                    </ItemTemplate>                    
                </asp:TemplateField> 
                <asp:TemplateField HeaderText="Roll No">  
                    <ItemTemplate>  
                        <asp:Label ID="lbl_RollNo" runat="server" Text='<%#Eval("RollNo") %>'></asp:Label>  
                    </ItemTemplate>                    
                </asp:TemplateField> 
                <asp:TemplateField HeaderText="Student Name">  
                    <ItemTemplate>  
                        <asp:Label ID="lbl_SName" runat="server" Text='<%#Eval("SName") %>'></asp:Label>  
                    </ItemTemplate>                    
                </asp:TemplateField>
                <asp:TemplateField HeaderText="Father Phone">  
                    <ItemTemplate>  
                        <asp:Label ID="lbl_FPhone" runat="server" Text='<%#Eval("FPhone") %>'></asp:Label>  
                    </ItemTemplate>                    
                </asp:TemplateField>
                 <asp:TemplateField>
                        <HeaderTemplate>Status</HeaderTemplate>
                         <ItemTemplate>
                            <asp:Label ID="lbl_Status" runat="server" Text="Absent"></asp:Label>  
                         </ItemTemplate>
                    </asp:TemplateField>                
            </Columns>              
        </asp:GridView>

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            this.BindGrid();
            DateTime ADate = DateTime.Now;
            txtDate.Text = ADate.ToString("dd.MM.yyyy") + " " + ADate.DayOfWeek.ToString();
        }
    }
    protected void BindGrid()
    {
        GridView1.Visible = true;
        con = new SqlDbConnect();
        con.SqlQuery("select AdmissionNo,RollNo,SName,FPhone from tblStdReg where Active_Status='Active' order by RollNo;");
        adapt.SelectCommand = con.Cmd;
        adapt.Fill(sTable);
        if (sTable.Rows.Count > 0)
        {
            GridView1.Visible = true;
            GridView1.DataSource = sTable;
            GridView1.DataBind();
            lblTotal.Text = GridView1.Rows.Count.ToString();
        }
        else
        {
            GridView1.Visible = false;
            ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('No Record Found');", true);
            lblTotal.Text = "0";
            return;
        }
        con.conClose();
    }
    protected void btnAtten_Click(object sender, EventArgs e)
    {
        int i = 0;
        GridViewRow row = GridView1.Rows[i];

        con = new SqlDbConnect();
        con.SqlQuery("select AdmissionNo from tblStdReg where AdmissionNo=@AdNo");
        con.Cmd.Parameters.Add(new SqlParameter("@AdNo", this.txtRegNo.Text.Trim()));
        con.RdrEx();
        if (con.Rdr.Read())
        {
          string lblStatus = (row.FindControl("lbl_Status") as Label).Text;
          lblStatus = "Present";
          Response.Write("This"+txtRegNo.Text+"-"+lblStatus);
        }
        else 
        {
            Response.Write("Reg. No. not found Try again");
        }
    }