You will need to
Go into your extension dir and remove the civivolunteer code that was
installed with your buildkit.
Then git clone their master (to get the latest).
Then go to the Manage Extensions page to check if there are any
prompts for database updates.
Then disable a small other extension and re-enable it. Or just enable
another new extension via ...
CiviCRM 4.2.4 is a very old version - it's from 2012. It's not compatible with newer versions of PHP - I'd be surprised if it worked with PHP 5.6, let alone 7+. You can downgrade PHP or upgrade CiviCRM - but since you're upgrading from such an old version, be sure to a) take a backup of the database before beginning, b) upgrade a few versions at a time (I'...