There's a couple of tools people have written for other XNA games e.g. http://gamedev.stackexchange.com/questions/5840/xnb-file-reader - google for others. Many of them are unreliable or old so good luck.
The format is documented here if you want to write your own http://xbox.create.msdn.com/en-US/sample/xnb_format
It can be tough to get back to the original file formats though because most of the data is converted to something friendlier for the game to load so for example its no longer in JPG or PNG or FBX format so you can't simply save it out.