Click the PowerPress link in the left-hand side of your WordPress dashboard. At the bottom of the page, you'll see a checkbox for Custom Podcast Channels. Click the checkbox and then click the Save Changes button.
Now, you should see a link for Podcast Channels in the left-hand menu, click it. On the next page, you should see your podcast listed there with an Edit link underneath. Click the edit link and go to Feed Landing Page URL. Enter the new URL you want to appear in your iTunes Store page and click the Save Changes button.