Without Plugin:

1. Add constant to wp-config.php

define(\'WP_ADMIN_DIR\', \'admin-area\');
define(\'SITECOOKIEPATH\', preg_replace(\'|https?://[^/]+|i\', \'\', get_option(\'siteurl\') . \'/\' ) );
define(\'ADMIN_COOKIE_PATH\', SITECOOKIEPATH . WP_ADMIN_DIR); 

2. Add below filter to functions.php

add_filter(\'site_url\',  \'wpadmin_filter\', 10, 3);  

function wpadmin_filter( $url, $path, $orig_scheme ) {  
    $old  = array( "/(wp-admin)/");  
    $admin_dir = WP_ADMIN_DIR;  
    $new  = array($admin_dir);  
    return preg_replace( $old, $new, $url, 1); 
}

3. Add below line to .htaccess file

RewriteRule ^admin-area/(.*) wp-admin/$1?%{QUERY_STRING} [L]
RewriteRule ^admin-area/?$ /wp-login.php [QSA,L]
RewriteRule ^admin-area/register/?$ /wp-login.php?action=register [QSA,L]
RewriteRule ^admin-area/lostpassword/?$ /wp-login.php?action=lostpassword [QSA,L]

Also you can redirect Using this function

add_action(\'login_form\',\'redirect_wp_admin\');

    function redirect_wp_admin(){
    $redirect_to = $_SERVER[\'REQUEST_URI\'];
        if(count($_REQUEST)> 0 && array_key_exists(\'redirect_to\', $_REQUEST)){
        $redirect_to = $_REQUEST[\'redirect_to\'];
        $check_wp_admin = stristr($redirect_to, \'wp-admin\');
                if($check_wp_admin){
                wp_safe_redirect( \'404.php\' );
                }
        }
    }

NOTE : 在启动激活插件之前备份数据库。在开始激活插件之前备份数据库是非常重要的。