If a title you already own has been revised/updated, update the edition on your Kindle device using this procedure:
Optional: You can elect to have future updates automatically delivered to your Kindle in Preferences:
If you are not seeing a title you purchased listed (or it does not have an update button but you know you don't have the latest version), then contact Amazon Support.
They will be able to update it for you manually (which requires you reach out to support).
Hope that helps!
PS: Additional info is here