Open the file menu, and choose export audio. That'll open a regular dialog like you'd get in any word processor or whatever, where you can give it a name, choose the file type, such as wav, mp3, etc, and choose where you want to save to. You might also get a dialog asking for metadata such as artist name and stuff. You can leave this blank or fill it in, it's up to you. There's a checkbox in preferences>import/export to turn off this prompt if you never bother filling it in.