Excel Issue

In Crosstab report, I’m facing an issue where I’m able to download an Excel file, but the changes I made to the file through code aren’t showing up in the downloaded version.
In page render, I’m changing the field caption using the below code,

// Page Render event
function Page_Render()
{
    if (isset($_GET['x_PLAN_FY']))
        $_SESSION['fy_ctb'] = $_GET['x_PLAN_FY'];
    else
        $_SESSION['fy_ctb'] = NULL;
    if (isset($_GET['x_PLAN_TYPE']))
        $_SESSION['plan_type'] = $_GET['x_PLAN_TYPE'];
    else
        $_SESSION['plan_type'] = NULL;
    if (isset($_GET['x_ORDERNO'])){
        $_SESSION['orderno_ctb'] = $_GET['x_ORDERNO'];
        $_SESSION['orderno_ctb'] = str_replace('|', ',', $_SESSION['orderno_ctb']);
    }
    else
        $_SESSION['orderno_ctb'] = NULL;
    $fy_start = substr($_SESSION['fy_ctb'], 2, 2);
    $fy_end = substr($_SESSION['fy_ctb'], -2);
    if (isset($_GET['x_ORDERNO'])){
        $order_nos = $_SESSION['orderno_ctb'];
        $plan_type = $_SESSION['plan_type'];
        $order_numbers = explode(',', $order_nos);
        foreach ($order_numbers as $order_no) {
            $order_no = trim($order_no);
            $plan_year = ExecuteScalar("SELECT DISTINCT PLAN_YEAR FROM sc_team_wise_occupancy_actual_res_view WHERE ORDERNO = $order_no AND PLAN_TYPE = '$plan_type'");
            $plan_year = substr($plan_year, 2, 2);
            $plan_month = ExecuteScalar("SELECT DISTINCT PLAN_MONTH FROM sc_team_wise_occupancy_actual_res_view WHERE ORDERNO = $order_no AND PLAN_TYPE = '$plan_type'");
            $months = ["January", "February", "March","April", "May", "June", "July", "August", "September", "October", "November", "December"];
            $monthIndex = $plan_month - 1;
            $month = $months[$monthIndex];
            $month = substr($month, 0, 3);
            for($i = 1; $i <= $this->ColumnCount; $i++) {
                $Column = $this->Columns[$i];
                if ($Column->Caption != "") {
                    if ($Column->Caption == $order_no)
                        $Column->Caption = $month."-".$plan_year;
                }
            }
        }
    }
    if (isset($_GET['x_PLAN_FY'])){
        for($i = 1; $i <= $this->ColumnCount; $i++) {
            $Column = $this->Columns[$i];
            if ($Column->Caption != "") {
                $months = [
                    "1" => "Apr",
                    "2" => "May",
                    "3" => "Jun",
                    "4" => "Jul",
                    "5" => "Aug",
                    "6" => "Sep",
                    "7" => "Oct",
                    "8" => "Nov",
                    "9" => "Dec",
                    "10" => "Jan",
                    "11" => "Feb",
                    "12" => "Mar"
                ];
                $monthIndex = $Column->Caption % 12;
                if ($monthIndex == 0) {
                    $monthIndex = 12;
                }
                if ($monthIndex == 10 || $monthIndex == 11 || $monthIndex == 12) {
                    $Column->Caption = $months["$monthIndex"] . "-" . $fy_end;
                } else {
                    $Column->Caption = $months["$monthIndex"] . "-" . $fy_start;
                }
            }
        }
    }
}

this changed captions not coming in excel while downloading.