Note: If you need to replace the audio for an already published episode, do not delete the episode. 

1) Under the Podcast tab, navigate to the show you need to change audio for. Click to open the episode.

2) Once on the Episode Page, click the "Edit Episode" button in the top right corner.
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3) On the Edit Episode page, you can push the "Upload" button to add a new audio file. Click the green "Save" button.
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4) Once the audio has been saved, you will see a the audio processing bar.
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After the audio has finished processing, you will have to reset ad insertion points since it is an entirely new audio file. Check out this article for more information on setting ad locations.