Under Mac OS X you can change file encoding with TextWrangler (Free)
You can also do with Terminal.app and the command line :
iconv -f iso-8859-1 -t utf-8 < file.txt > new_file.utf-8.txt
PS: Assuming to original file is in iso-8859-1 and you want to convert it to utf-8. See iconv manual page for more details.