edit

<?php
    include 'lib/library.php';

    if ($_SERVER['REQUEST_METHOD'] == 'POST') {
        $nis            = $_POST['nis'];
        $nama_lengkap   = $_POST['nama_lengkap'];
        $jenis_kelamin  = $_POST['jenis_kelamin'];
        $kelas          = $_POST['kelas'];
        $jurusan        = $_POST['jurusan'];
        $alamat         = $_POST['alamat'];
        $goldar         = $_POST['goldar'];
        $ibu            = $_POST['ibu'];

        $sql = "UPDATE siswa SET nis = '$nis', nama_lengkap = '$nama_lengkap', jenis_kelamin = '$jenis_kelamin', kelas = '$kelas', jurusan = '$kelas', alamat = '$alamat', goldar = '$goldar', ibu = '$ibu' WHERE nis = '$nis' ";
        $mysqli->query($sql) or die ($mysqli->error);
        header('location: index.php');
    }

    $nis = $_GET['nis'];

    if (empty($nis)) header('location: index.php');

    $sql = "SELECT * FROM siswa WHERE nis = '$nis' ";
    $query = $mysqli->query($sql);
    $siswa = $query->fetch_array();

    if (empty($siswa)) header('location: index.php');

    include 'views/v_tambah.php';