Each of the major online retailer handles content updates a little differently:

• Amazon: Some Kindle devices will automatically update the eBook file, but readers have the option to turn this off. If Amazon considers the changes to be "significant" (i.e., not simply fixed typos), they will notify customers about the update.
  

• Apple: eBook updates are handled just like app updates. Previous customers are able to get your changes.
  

• Barnes & Noble, Kobo, Google Play: There is not a way for previous customers to get updates to your eBook.