You could try editing it using Sigil (there's a Sigil forum
here in MobileRead where you can find links to download and install Sigil, and lots of info about using it). Make a copy of the EPUB file first, though, just in case it doesn't turn out the way you want it to!
If you know HTML and CSS, an alternative would be to change the file extension from .epub to .zip (again, make a copy of the EPUB file first!), then unzip the file and hand edit the (X)HTML content files and the CSS file(s). Note that when you zip the edited files up again, you need to ensure that the mimetype file is the first file in the zip archive, and uncompressed.