When the XIB is selected, and you click on your window, you'll see
Placeholders, File's Owner, First Responder,
Objects, App Delegate, Window...
I had duplicated a project where I had items set up properly and changed the name of my AppDelegate .h and .m files and the name of the @Interface in each to be more appropriate to the project.
Of course, when I ran this iOS app in the simulator, it failed right away. Teh Googles told me the relationship I needed to understand that that my App Delegate in the xib file also had to be renamed.
That was what I was trying to do. Change the name of the App Delegate from the old one to the new one in the interface editor for xib files.
It doesn't let you. At least not as far as I can tell. I ended opening up the xib in an XML editor and did a search and replace.