First you need to figure out what encoding your file really is written in.
"Unicode" is not an encoding for file data, it's just a numbering system for characters. UTF-8 is one of several possible ways to encode unicode data for storage in files. Other popular ones are UTF-16 and UTF-32, both of which exist in big-endian and little-endian versions.
As long as you just know "unicode", your guess is as good as mine about the actual contents of the file, and the best way to convert it.