You might be better starting from scratch.
Upgrade to 2015.4
Use Web as the intent
Export to ePub version 3
Derek, thank you for your thoughts!
Do I understand correctly that after upgrade, I will open the ready INDD layout for a print book and set the intent to Web?
As the book has already been designed (for print, though) - designing from scratch doesn't seem reasonable.
It's impossible to answer this without seeing the layout. If it's simple text in one flowing story it should be fine, but if you have multiple text frames, graphics and a complicated layout, you may well be better off starting from scratch with a new layout.
And Derek is right...use version 3. There's almost no reason I can think of to use 2.
The first thing to do is export the existing book to Reflowable text ePub, open the result in a viewer or two and see what the resulting ePub looks like. You can then see what needs to be done. Depending on the kind of design and content it may be fine or need a couple of tweaks or it may be better to completely redo the book for ePub.
Thank you, Bob! I will give it a try with ePub v3. ePubs that were made for us were in v2, therefore we thought we should stick to it.
Most of our books are fiction, with simple layout.
So I only need to change the intent to Web if I want to start over with design? Or it is better to change it to Web for all layouts that I plan to export to epub?
The only thing the intent does is give you a different set of presets for document size and changes the units to pixels. It has no real effect on the work you’re doing.
I would add that it’s relatively easy to produce poor reflowable text ePubs. Creating ePubs is quite different from producing documents for printing, it’s not intuitive and it takes some effort. I strongly recommend you get some tuition. The best I know are the online video tutorials on Lynda.com, such as https://www.lynda.com/InDesign-tutorials/InDesign-CC-2015-EPUB-Fundamentals/374187-2.html
You can get a 10-day free trial.
You dont need to change the Intent in "Document Setup" from Print to Digital. Just right click in the rules and change the unit to pixels - in both rules (vert/horz). Changing the intent in most of the times change the page size and mess the layout.