Inside the episode, you have the option to replace the audio/video file that's delivered to listeners. The uploaded file will be converted to mp3 if it's not already an mp3. The same file types and size limits are enforced here as the main upload area of a feed.

In some cases, an upload appears stuck and won't let you publish the episode. You can replace the file to give it another shot at uploading/converting. This may happen if you lose internet connection or your computer goes to sleep before the upload finishes.

Be sure to keep the computer online until the upload finishes. Refresh the page to see if the episode is ready for publishing.