The folder structure of Drupal 8 is quite different than the Drupal 7 version. You have to know all about this structure before updating your Drupal 8 site.
Put your site in maintenance mode
Use an FTP program or use the Control Panel
First make a backup of your site.
A backup with Backup and Migrate is on this moment not possible. You have to do it with the Control Panel.
The last step of the databse backup..
In the filemanager you see the folder structure. There are only two folders thas has to be upgraded: Core and Vendor. Stay away from the rest. The files (from autoload.php to web.config) must be deleted too. They have to be replaced. Keep in mind that the module folder is completely different from the Druapl 7 version. It contains now the modules that have been installed after the core Drupal installation.
Delete Core, Vendor and the root files. Replace them with the new version.
After that go to sitename.ext/update.php. This is the database control for the new version. Ready...