Adding a special tag to the old feed

Hello everyone. For some crazy reason I gave itunes and Stitcher my Powerpress feed. Which was a mistake. I don't want to do that. I have Libsyn and that's what I want to use to upload and send out my podcast. Now whenever I upload a new episode in Libsyn, itunes and stitcher cant find it. Why, because I gave them powerpress as my feed. I want to just get rid of powerpress all together.

Now I've been reading that I need to add a special tag to the old feed which would be powerpress (inside my wordpress blog) But I don't know where I'm supposed to add this tag at? Can anyone help me with this? I'm told the tag would be: <itunes:new-feed-url>

If you could direct me to some screen shots or a video on where and how to put this in, I would greatly appreciate it.

Thanks

Why ATL