Step 1
Following update information at Symfony / Sensio Labs website:
https://symfony.com/doc/current/setup/upgrade_major.html
and changed composer.json file.
But DO NOT change
"symfony/web-server-bundle": "4.4.*",
Step 2
I could not update due to error message:
The requested package nelmio/cors-bundle
(locked at 1.5.6, required as ^2.0) is satisfiable by
nelmio/cors-bundle[1.5.6] but these conflict with your
requirements or minimum-stability.
I had to change the file composer.lock from
"name": "nelmio/cors-bundle",
"version": "1.5.6",
to
"name": "nelmio/cors-bundle",
"version": "2.0.1",
Step 3
Last error message was
Uncaught Error: Class 'Symfony\Component\Debug\Debug' not found
and could be fixed by information from
https://github.com/symfony/recipes/blob/master/symfony/framework-bundle/4.4/public/index.php
changing public/index.php
from
use Symfony\Component\Debug\Debug
to
use Symfony\Component\ErrorHandler\Debug;