The feed URL can only be edited if Google Reader sync is off.
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.