My computer tries to open .acsn files with word. How do I change that?

I have Windows 8.1. I just downloaded Adobe Digital Editions 4.0. The first book I tried to download from my library, when it asked me what to open the download in I accidentally clicked "word" which obviously didn't work but it wouldn't let me change it.  I downloaded a second book just to try to get it right, but now it doesn't give me a choice and automatically tries to open in word. I can't change how it tries to open .acsn files at all. Any advise would be greatly appreciated.