Can I change the URL for RSS feed in NetNewsWire?
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.