Upgrade from 8.2.1 to 9.0.1?

Currently using Concrete5 v. 8.2.1. with PHP v. 7.1.33 for a very simple website that I set up some years ago. It seems like I should upgrade for security’s sake. Where should I look to start that process?

You can start here How to Upgrade Concrete5

This has a lot of info on security configurations Security :: Concrete CMS - Org

You are likely going to need to update your PHP version as well:

Additionally, I would add that you’ll very likely want to update to the latest stable version of 8, before you update to version 9. Late versions of 8 prepare your site for the version 9 updates and so it’s usually best to be on a high version of the previous major version before attempting the major version update.

Additionally, by updating to the latest stable version of Concrete CMS 8 you’ll apply a number of security updates as well.