From time to time there may be need to update your Apple Podcasts RSS feed due to updated metadata like a new podcast cover. In general, this gets updated across the board when a change is made on your hosting platform (e.g. Podcast.co), but Apple Podcasts typically requires a manual refresh.
First, log in to Apple Podcast Connect and open your podcast.
Next, click the refresh button.
It may take up to a couple days to reflect the changes, but otherwise that's it!
To confirm it's been updated, reload the podcast on your various devices (particularly in incognito mode or on a device that hasn't loaded the podcast yet).
If after a few days the artwork/metadata isn't showing up accurately, you'll want to contact Apple Podcast support directly via Podcast Connect's contact page.