Start in the Podcasts Library and click on your podcast. 

From this page, click on "Edit Podcast" in the top right:

Find the "Rating" box down on the page and select the appropriate rating, either Clean or Explicit.

You can also change an individual episode’s rating by clicking on an episode and selecting “Edit Episode” in the top right corner. You will see an Episode Rating box where you can change individual episodes to Clean or Explicit based on content for that specific episode.