The short answer is that, in many cases, you don’t need to change anything. And, you most certainly do not want to simply migrate all of the contents of viewDidUnload to didReceiveMemoryWarning.
Generally, most of us do the setting of IBOutlet references to nil in viewDidUnload (largely because Interface Builder would put that there for us) and do the general freeing of memory (e.g. clearing of caches, releasing of easily recreated model data, etc.) in didReceiveMemoryWarning. If that’s the way you do it, then you probably don’t require any code changes.
According to the iOS 6 viewDidUnload documentation:
Views are no longer purged under low-memory conditions and so this method is never called.
Therefore, you do not want to move the setting of your IBOutlet references to nil anywhere, because the views are no longer purged. It would make no sense to set them to nil in didReceiveMemoryWarning or anything like that.
But, if you were responding to low memory events by releasing easily-recreated model objects, emptying caches, etc., in viewDidUnload, then that stuff should definitely move to didReceiveMemoryWarning. But then, again, most of us already had it there already.
Finally, if you free anything in didReceiveMemoryWarning, just make sure your code doesn’t rely upon them being recreated in viewDidLoad again when you pop back, because that will not be called (since the view, itself, was never unloaded).
As applefreak says, it depends upon what you were doing in viewDidUnload. If you update your question with explicit examples of what you had in your viewDidUnload, we can probably provide less abstract counsel.