If you're using Blubrry media hosting, log in to your dashboard at blubrry.com and click the Upload New Media link. Upload the new version of the file using the EXACT SAME filename as the old version. Once it's finished uploading, click the Make Public link. This'll overwrite the old version of the file with the new version. It may take a few hours for the new version to propagate across Blubrry's CDN but it will get there.