How do I update CiviVolunteer, from the UI or from the command line?
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 the Manage Extensions page. This will
trigger managed entity updates.