saving edited row value back to inmemmory datatable

protected void btnFixZero(object sender, EventArgs e)
      {
          System.Data.DataTable dt = new System.Data.DataTable();
          dt = (System.Data.DataTable)ViewState["SelectedRecords"];
          int rowscount = GridView2.Rows.Count;
          int columnscount = GridView2.Columns.Count;
          if (missingCell == 0)
          {
              foreach (DataRow row in dt.Rows)
              {

                  string cellData = row["F4"].ToString();
                  string sub0 = cellData.Substring(0, 1);
                  string sub1 = cellData.Substring(1, 1);
                  string subplus = cellData.Substring(0, 1);

                  if (sub0 == "2" && sub1 == "7")
                  {
                      cellData = cellData.Remove(0, 2);
                      cellData = "0" + cellData;
                      row["F4"] = cellData;//had to specify which column
                      row.AcceptChanges();
                      dt.AcceptChanges();
                  }
                  if (sub0 != "0")
                  {
                      cellData = "0" + cellData;
                      row["F4"] = cellData;//had to specify which column
                      row.AcceptChanges();
                      dt.AcceptChanges();
                  }

              }
              dt.AcceptChanges();
              ViewState["SelectedRecords"] = dt;


              GridView2.DataSource = dt;
              GridView2.DataBind();
          }

             }