Just change the details in the feed. Regarding the image, make sure to give it a different file name or the Store (which caches the images) won't know it's changed. Then just republish the feed. Subscribers will see the changes pretty well immediately but the Store may take several days to reflect the changes. You don't have to take any other action. The URL of your Store page (found by control-clicking on the image in the Store and choosing 'Copy URL') will change to reflect the new title but that part of the URL doesn't matter anyway - you can leave it out or change it to 'Fred' and the URL will still work - it's the number which is the identifier.