1. composer.json:

"require": {
        "php": ">=7.0.0",
        "fideloper/proxy": "~3.3",
        "laravel/framework": "5.5.*",
        "laravel/tinker": "~1.0"
    },

To:

"require": {
        "php": ">=7.1.3",
        "fideloper/proxy": "~4.0",
        "laravel/framework": "5.6.*",
        "laravel/tinker": "~1.0"
    },

2. Replace app\Http\Middleware\TrustedProxies.php file with contents below:

<?php

namespace App\Http\Middleware;

use Illuminate\Http\Request;
use Fideloper\Proxy\TrustProxies as Middleware;

class TrustProxies extends Middleware
{
    /**
     * The trusted proxies for this application.
     *
     * @var array
     */
    protected $proxies;

    /**
     * The headers that should be used to detect proxies.
     *
     * @var string
     */
    protected $headers = Request::HEADER_X_FORWARDED_ALL;
}

3. composer update

https://stackoverflow.com/questions/48594851/upgrading-laravel-5-5-to-5-6-error

Official:
Upgrading To 5.6.0 From 5.5 https://laravel.com/docs/5.6/upgrade

Mylog:

C:\server\xampp\htdocs\projects\laravel5\auction3>php artisan –version
Laravel Framework 5.5.21

C:\server\xampp\htdocs\projects\laravel5\auction3>composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 3 installs, 40 updates, 1 removal
– Removing mtdowling/cron-expression (v1.2.1)
– Updating symfony/css-selector (v3.3.13 => v4.1.0): Loading from cache
– Updating tijsverkoyen/css-to-inline-styles (2.2.0 => 2.2.1): Loading from cache
– Updating symfony/polyfill-mbstring (v1.6.0 => v1.8.0): Loading from cache
– Updating paragonie/random_compat (v2.0.11 => v2.0.15): Downloading (100%)
– Updating symfony/http-foundation (v3.3.13 => v4.1.0): Downloading (100%)
– Updating symfony/event-dispatcher (v3.3.13 => v4.1.0): Loading from cache
– Updating symfony/debug (v3.3.13 => v4.1.0): Downloading (100%)
– Installing symfony/polyfill-ctype (v1.8.0): Loading from cache
– Updating egulias/email-validator (2.1.3 => 2.1.4): Loading from cache
– Updating ramsey/uuid (3.7.1 => 3.7.3): Loading from cache
– Updating psr/simple-cache (1.0.0 => 1.0.1): Loading from cache
– Updating symfony/translation (v3.3.13 => v4.1.0): Loading from cache
– Updating nesbot/carbon (1.22.1 => 1.25.0): Loading from cache
– Updating league/flysystem (1.0.41 => 1.0.45): Loading from cache
– Updating erusev/parsedown (1.6.4 => 1.7.1): Loading from cache
– Updating doctrine/inflector (v1.2.0 => v1.3.0): Loading from cache
– Installing symfony/polyfill-php72 (v1.8.0): Loading from cache
– Updating symfony/var-dumper (v3.3.13 => v4.1.0): Downloading (100%)
– Updating symfony/routing (v3.3.13 => v4.1.0): Downloading (100%)
– Updating symfony/process (v3.3.13 => v4.1.0): Downloading (100%)
– Updating symfony/http-kernel (v3.3.13 => v4.1.0): Downloading (100%)
– Updating symfony/finder (v3.3.13 => v4.1.0): Downloading (100%)
– Updating symfony/console (v3.3.13 => v4.1.0): Downloading (100%)
– Installing dragonmantank/cron-expression (v2.2.0): Downloading (100%)
– Updating laravel/framework (v5.5.21 => v5.6.25): Downloading (100%)
– Updating fideloper/proxy (3.3.4 => 4.0.0): Loading from cache
– Updating intervention/image (2.4.1 => 2.4.2): Loading from cache
– Updating nikic/php-parser (v3.1.2 => v4.0.2): Loading from cache
– Updating psy/psysh (v0.8.15 => v0.9.6): Downloading (100%)
– Updating laravel/tinker (v1.0.2 => v1.0.7): Loading from cache
– Updating laravelcollective/html (v5.5.1 => v5.6.9): Downloading (100%)
– Updating filp/whoops (2.1.14 => 2.2.0): Loading from cache
– Updating sebastian/comparator (2.1.0 => 2.1.3): Loading from cache
– Updating doctrine/instantiator (1.0.5 => 1.1.0): Loading from cache
– Updating phpunit/phpunit-mock-objects (4.0.4 => 5.0.7): Loading from cache
– Updating phpunit/php-file-iterator (1.4.3 => 1.4.5): Loading from cache
– Updating phpunit/php-token-stream (2.0.1 => 2.0.2): Loading from cache
– Updating phpunit/php-code-coverage (5.2.3 => 5.3.2): Loading from cache
– Updating webmozart/assert (1.2.0 => 1.3.0): Loading from cache
– Updating phpdocumentor/reflection-docblock (4.1.1 => 4.3.0): Loading from cache
– Updating phpspec/prophecy (1.7.3 => 1.7.6): Loading from cache
– Updating myclabs/deep-copy (1.7.0 => 1.8.1): Downloading (100%)
– Updating phpunit/phpunit (6.4.4 => 6.5.8): Loading from cache
Writing lock file
Generating optimized autoload files
> Illuminate\Foundation\ComposerScripts::postAutoloadDump
> @php artisan package:discover
Discovered Package: fideloper/proxy
Discovered Package: intervention/image
Discovered Package: laracasts/flash
Discovered Package: laravel/tinker
Discovered Package: laravelcollective/html
Package manifest generated successfully.

C:\server\xampp\htdocs\projects\laravel5\auction3>php artisan –version
Laravel Framework 5.6.25

C:\server\xampp\htdocs\projects\laravel5\auction3>