Instead on doing such long and hectic procedure it better to learn and update drupal 8 core through DRUSH.

1. Install Drush for Drupal8. (Check how to install Drush on shared hosting for drupal)

2. Put your site in maintenance mode through drush.

 drush sset system.maintenance_mode 1

3. Run update command in drush to start update.

drush pm-update

4. Put your site online again

drush sset system.maintenance_mode 0

You can find more detailed explanation at source Update Drupal 8 core with Drush.

Instead on doing such long and hectic procedure it better to learn and update drupal 8 core through DRUSH.

1. Install Drush for Drupal8.

2. Put your site in maintenance mode through drush.

 drush sset system.maintenance_mode 1

3. Run update command in drush to start update.

drush pm-update

4. Put your site online again

drush sset system.maintenance_mode 0