How to upgrate drupal 8
Before any changes in your current installation you must make copies of the database and the roor folder of drupal.
Download the latest version in .zip or .tar.gz format from drupal.org/download
In the root of your installation remove the folders core, vendor and all the files in the root directory except gulpfile.js and package.json (remove hidden files too)
Extract the compressed file from the new downloaded version in the root folder of the installation. Be aware of the hidden files that you have to extract too.
In your website (while logged in as admin) run /upgrade.php. This will update database schema and make all the changes.
Lastly, check in administration of your website on /admin/modules and admin/modules/updates to confirm the new version of the core and the modules.