When you download a book, you create an account that gives you access. Simply log in again and you will have access to all your updated products. Go into each product and download the newest version. 
I usually include the date in the file name so you can easily see if you have the latest version or not.

Here are some options:

Then don't read it! Just go through and look at all the videos and pictures. 
You won't remember ever single screenshot and video (if you can, I'll be super impressed) but you will get an idea of the variety of everything you can do. When you come to a SwiftUI problem, you will remember the video/screenshot. Go to the book and open it and get the code you need.

Minor Updates
You will get free updates for minor revisions made to this book. If you opted in to receive emails, you will get a notification when the book is updated and you can re-download the updated version.
Major Rewrites
If there is a significant rewrite of the book due to technological changes then you will get a large discount to upgrade to the rewritten book.