How to open .epub file?

there are many out there, but how about iBooks ?

 

iBooks is OK, you can use also FBreader or something from the list placed here
http://fileextension.info/file/epub

 

Do you mean to read or to edit? If you want to look at the component files, a .epub file is a zip archive. You can change the extension to .zip and use any unzip tool. I use Oxygen as my ebook editor, but unless you are doing this every day, or can get the academic license, it is a costly option. Calibre can also do this. This is presuming no DRM on the file. In that case, consult Mr. Google.