Similar to this question, but would like to know specifically if there's a way to do this through the iBookstore/iTunes Producer, and what the process might be.
The books are epub2 and epub3, not iBooks Author.
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Yes, this happens automatically.
During the publishing process, you specify whether the book is new or an update to a previously published version. If it is an update, you must provide a new version number. Readers who downloaded your book are notified in iBooks that a new version is available.