header already sent

ane mau update, update bisa tapi ada bacaan gini gan.. Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\alfattah\Admin\sidebar.php:102) in C:\xampp\htdocs\alfattah\Admin\visi_misi.php on line 13

gimana ya cara ngatasinnya ??

scriptnya gini gan

<?php
include "header.php";
include "sidebar.php";
include "koneksi.php";
?>
<?php
 if(isset($_POST['submit'])){
    $isi_profil           = $_POST['isi_profil2'];

  $update = mysqli_query($koneksi, "UPDATE tbl_profil SET isi_profil='$isi_profil' WHERE id_profil ='2'") or die(mysqli_error($koneksi)); // query untuk mengupdate nilai entri dalam database

    if($update){ // jika query update berhasil dieksekusi
    header('location:sejarah.php');
      }else{ // jika query insert gagal dieksekusi
      echo '<div class="alert alert-danger alert-dismissable"><button type="button" class="close" data-dismiss="alert" aria-hidden="true">×</button>Ups, Data Gagal Di simpan!</div>'; // maka tampilkan 'Ups, Data kegiatan Gagal Di simpan!'
       }
    }
?>
<?php
$sql = mysqli_query($koneksi,"SELECT isi_profil FROM tbl_profil WHERE id_profil='2'");
$data = mysqli_fetch_array($sql);
?>