Transcoder is "a library of Java classes designed to translate Ancient Greek from one encoding to another."
You can download a .jar that can be used as a stand-alone program if you have a Java Runtime installed. Once you open the program, you can select the input and output encodings from the Encodings menu, then paste/copy some text and click on Transform. Or you can select an input file and an output file from menu File - Set Set Source File and File - Set Set Result File.
Supported encodings include: