Can I continue to edit, improve or add to content I’ve already published?
Yes, you can. Whatever is your creation is yours, that is, yours to copyright. You can continue to add to it, improve and change it indefinitely without impacting your ownership of the work. However, as a practical matter, if readers are buying one version, you risk frustrating them if a week later, there’s a new, improved version available, right?
Adding and improving your written creative works is more the province of blogging or article writing rather than fixing what you’ve done in a book.