To change the episode description (or "show notes") for an episode, you'll first want to click on the episode on your podcast dashboard. Next, visit the Edit tab.
If you scroll down on this tab, you'll find that you can edit your show notes.
From this section, you can also edit episode auto-includes. Auto-includes options automatically add links or text to your show notes, and are kept up-to-date automatically. New episodes automatically get the same auto-include settings as your most recently uploaded episode.
After editing your episode, be sure to click Save at the bottom of the page.