Select the feed in the left column and press ⌘I to bring up the feed info window.  Click on the triangle next to the _URLs_ entry and you will get a text box that will allow you to edit the feed URL.  There are a bunch of other setting there that allow you to do things like specify how often a particular feed is updated, how long articles are kept, etc.

Edit: here is an example.
 
 ![NNW Window][1]


  [1]: https://i.sstatic.net/hYFzy.png