I don't recommend you use heuristics to solve this - it will change other things about your ebook which you don't want changed. Heuristics are intended to "guess" their way to a better structure and will do nasty things if you have the wrong options ticked - if the structure is fine to begin with you will end up with something worse than what you started.
The suggestion by theducks is spot on for 99% of these cases. Just open the conversion dialog, at the top choose to do an epub to epub conversion, click on the "Look & Feel" tab, and in the fifth box down where it says "Input character encoding" change it to "cp1252" and do your conversion.
There are still some circumstances where that won't fix the issue, usually because someone has mangled things even further along the way. In that scenario I use Tweak ePub, open all the html files up in Notepad++, do a bulk find/replace across the files after fiddling with the encodings and saving them as utf-8. It's a pain but it is the minority of times this needs to be done.