Upgrading
A: You don't
The only supported way to ensure you are on the latest scripts is to run the uninstaller,
pivpn uninstall
then run the installer again
curl -L https://install.pivpn.io | bash
Let me explain my logic.
First, this is really an installer. By definition alone an installer is used to install something, then it's done. The only time you'd need to update an installer is if you were installing again. Hence, uninstall the old one first.
Second, pivpn does provide a few management commands that are updated to resolve bugs. So it would be nice to update those without re-installing. However there is a big gotcha here. Those commands add and revoke certs. To add a cert you are also building the OVPN file for the client. I have and may yet still add configuration options to the client. When I do that, there may be corresponding options needed in the server. If this happens and you upgrade, all your old clients are hosed and you need to revoke their old certs and generate new ones. At that point you really might as well have uninstalled and re-installed cleanly.
My final argument is that it would take a lot of logic to code in an upgrader without breaking something and introducing its own set of bugs. IE see my second point above. Once you run into issues, it is a 5-10 min task to uninstall and reinstall. That really is the easiest path to victory in almost any situation. The only downside to a clean reinstall is re-distributing the client certificates (which depending on what is upgraded might have to happen anyway). I can safely assume that most of the home users leveraging this script will have a handful of clients AT MOST.
So my advice here is once you've used the installer, if you have clients working, leave as-is. Check back in a month or so and see if features/fixes were added you think you'd want. If they look like something you want or you are running into an issue you see is fixed, take the 10min to an hour to uninstall, re-install, and redistribute new client ovpn configurations.