// Using the shell
Navigate into your Drupal installation.
cd /path/to/your/drupal/directory
Remove the 'core' and 'vendor' directories.
rm -rf core vendor
Remove all of the files in the top-level directory.
rm -f *.* .[a-z]*
//Using an FTP client
Navigate to your Drupal directory.
Select all the files in the top-level directory (also hidden files starting with a dot) as well as the 'core' and 'vendor' directories, and delete them. Be sure not to delete the 'modules', 'profiles', 'sites', and 'themes' directories.
Optionally sometimes an update includes changes to default.settings.php file. This will be noted in the release notes.