You need to resubmit your book with the preview attached. Duplicate the book in iBooks Author using the file menu option 'Duplicate' and then save that second copy someplace. Make sure you don't edit the original file. Now go into the copy and decide which sections you want to remove. You leave in place only the pages you want available in the free sample preview. The length and choice of what to include in the sample is up to you. I recommend a complete chapter. If a novel, go for chapter one. If a textbook or guide, choose whichever chapter you feel is most appealing to the prospective buyer, in order to tempt them to buy.
That done, you export the copy to your desktop - don't use Publish.
Now begin the process of resubmitting your original, complete book using Publish in iBooks Author. When the iTunes Producer window pops up allowing you to fill in ISBN and other data, there's a tab where you'll find two square windows. In the left one is the actual book file being sent to Apple. You drag and drop (or click to find the file in your directories and choose) your preview sample into that second window.
You'll now see two iBooks icons in those windows, representing the sample and complete iBook.
When you then publish, iTunes producer will bundle preview and book together. It will report errors in both the book and sample, if there are any, at the time of upload.
I really recommend you do download the PDFs from the iTunes Connect website after logging in. The information is all there in the documentation Apple provides. These documents can appear intimidating in scope but focus on what you're looking for.