You can upload new audio for an episode at any time by visiting the episode's dashboard and clicking the Edit tab. Click the X to clear the episode's audio, upload a new audio file, and save.
Note that if your episode has already been published, your listeners will not re-download the new audio file if they have already downloaded the old audio file. To ensure these listeners get the new audio file, delete the existing episode and create the episode from scratch on the "New episode" page. Deleted episodes will not be removed from listeners devices, so this will produce a second episode, though it may be useful if the original audio file had missing content.