how to Delete and update the Row In gridView

    protected void btnSubmit0_Click(object sender, EventArgs e)
    {


        DataTable dt = new DataTable();

        dt.Columns.Add("Product");
        dt.Columns.Add("Product Name");
        dt.Columns.Add("Qunatity");
        dt.Columns.Add("UnitPrice");
        dt.Columns.Add("total");


        DataRow dr = null;
        if (ViewState["sale"] != null)
        {
            for (int i = 0; i < 1; i++)
            {
                dt = (DataTable)ViewState["sale"];
                if (dt.Rows.Count > 0)
                {
                    dr = dt.NewRow();

                    dr["Product"] = ddlProduct.SelectedValue;
                    dr["Product Name"] = ddlProduct.SelectedItem.Text;
                    dr["Qunatity"] = Convert.ToInt32(txtQuantity.Text);
                    dr["UnitPrice"] = Convert.ToInt32(TxtUnitPrice.Text);
                    dr["total"] = Convert.ToInt32(TxtTotal.Text);


                    dt.Rows.Add(dr);
                    GridView1.DataSource = dt;
                    GridView1.DataBind();

                }

            }
        }

        else
        {
            dr = dt.NewRow();

            dr["Product"] = Convert.ToInt32(ddlProduct.SelectedValue);
            dr["Product Name"] = ddlProduct.SelectedItem.Text;
            dr["Qunatity"] = Convert.ToInt32(txtQuantity.Text);
            dr["UnitPrice"] = Convert.ToInt32(TxtUnitPrice.Text);
            dr["total"] = Convert.ToInt32(TxtTotal.Text);


            dt.Rows.Add(dr);
            GridView1.DataSource = dt;
            GridView1.DataBind();

        }
        ViewState["sale"] = dt;


        lblItemAdd.Text = "item add in list";
        txtQuantity.Text = "";
        TxtUnitPrice.Text = "";
        TxtTotal.Text = "";

    }