This varies from retailer to retailer and device to device:

• Apple notifies readers within the iBooks app that there’s an updated version available. (It’s exactly like updating an app.)
   
• Amazon notifies customers that a new version is available if there is a significant change in the book’s contents. (This decision is made at Amazon’s discretion.) Some Kindle readers will automatically receive your new files; however, Kindle readers have the ability to opt out of receiving automatic book updates.
   
• Barnes & Noble, Kobo, and Google Play do not notify readers who have already purchased your book that a new version is available. Because these retailers don’t currently have a way to update the files for existing customers, only new readers will get your new files.