How to convert straight quotes to smart 'curly' typographer's quotes

I have an EPUB from a publisher that I'm doing some editing work on in Sigil. But it has been produced using straight quotes instead of smart quotes, same goes for the apostrophes.

I'm wondering if there is a way to change the XHTML files to smart quotes. It may be automatic in Word, but I'm not sure how to do it in an EPUB.

I use Sigil for editing.

Anyone have any ideas?