There are multiple ways to update your Drupal 8 installation but I prefer using composer for ease of use. However, if you have multiple PHP installations on your system, composer may use the wrong PHP version. Typically, you would update your Drupal 8 installation like this:
When making changes in your httpd-vhost.conf file, make sure to test it before reloading the apache config file or restarting apache.
Mac OSX
apachectl configtestXAMPP OSX
/Applications/XAMPP/xamppfiles/bin/apachectl configtestErrors should display, if there are any, otherwise you'll see the Syntax OK to indicate that there are no errors and you can reload the apache config file or restart apache.