title_body: Without ebook creation software, can I fix formatting issues on an epub?
upvoted_answer: Depending on what you mean by ebook creation software. You can always:
  
  
  Rename the epub to a zip (change the epub suffix of the filename to zip)
  Extract the zip file. 
  Manually edit the extracted HTML files with your favorite text editor - this assumes that you have an understanding of formatting with HTML
  Repackage the files/folder to zip again
  Rename the zip to epub again. (As user3019828 pointed out in a comment: you need to use a zipper that won't compress the mimetype file. For Windows, ePubPack is an option; for Mac, you can use ePub Zip.)
  
  
  The above steps does not involve any specific ebook creation software... But you should give a try to Calibre's heuristics fixing capabilities too. And Calibre also has a GUI to refine ebook contents...