Hello! In 2018 I purchased Rick Steves' Paris ebook. I am planning another trip, and I want to know if I can update mine for a lower price than a new book.
No, you can not.
Personally, no matter which brand of guidebook it is, I verify online with links to the sites, restaurants, etc. But this website also has an update section. But not in the way that I think you're asking.
Try ebay. Or see if your library has a newer edition.
Also, it's been my experience that newer editions don't change much in terms of main sites and itinerary suggestions. I believe they mostly update opening hours, hotels, restaurants, covid changes, etc. which you can and should verify online.
In addition, Rick Steves does not sell the ebook versions and has no control over their price.
Always have the latest book, the cost is the smallest part of what you'll spend on the trip. And you support Rick Inc.