Thanks very mush for the detailed observations, appreciate the time taken by you. I have made some changes as suggested. Of the 14 suggestions ie 1,2,4,9,14 I have changed as enclosed:
<?php 
session_start();
session_regenerate_id (true); 
setlocale(LC_MONETARY, 'en_IN');
date_default_timezone_set('Asia/Kolkata');
include "config.php"
?>
<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8"/>
    <title> FDMS Login : </title>
    <link rel="stylesheet" href="/css/mobile.css" media= "screen and (max-width: 768px)"  type="text/css" />
    <link rel="stylesheet" href="/css/login.css" type="text/css"/>
    
</head>
<body> 
        <header>FDMS Staff Login</header>
<?php
if ($_POST['login']) {
$username=trim($_POST['username']);
$password=trim($_POST['password']);
$query = "SELECT * FROM staff WHERE staff_name='$username'";
$result = mysqli_query($conn, $query) ;
while ($row=mysqli_fetch_array($result)) {
        $staff=$row['staff_name'];
        $password_hash=$row['password'];
        $userlevel=$row['level'];
        if (password_verify($password,$password_hash )) {
            $_SESSION['username'] = $username;
            $_SESSION['user_level'] = $userlevel;
            if ($userlevel==0) {      
            header("Location:attendence.php");
            exit();
            } 
            if ($userlevel==9) {
            header("Location:complaint.php"); 
            exit();
            } else {
                echo "invalid";
            }
    }
    }
}
?>   
    <main>
        <div class="form_login">
            <form method="POST" name="login" id="login">
            <div class="row">
            <input type="text"  name="username" placeholder="Username" autofocus="true"/>
            </div>
            <div class="row">
            <input type="password"  name="password" placeholder="Password"/>
            </div>
            <div class = "button">
            <input type="submit" value="Login" name="submit_pass"/>
            </div>
            </form>
        </div>
    </main>
                    
        <div class='bottom'>
            <a href='index.php'>Return to Website</a>
        </div>
    <footer>@ Deepak</footer>
    
    </body> 
</html>
the rest 4,5,6,7,8,11,12,13 I did not get
Thanks again