I don't understand what you are trying to do..
*.ini files are configuration settings and *.rar files are compressed archives
They are totally different, so even if you did change the extension (see near end of post), the file wouldn't be a rar file
Anyway, to change the file-extension, click "Tools->Folder Options" and untick "Hide known file extensions"
Now, simply rename the file, and don't forget the file extension!!