Sadly there’s no UI to change the type of a XIB from iPhone to iPad or vice versa. The only time you can select the type is during creation of the XIB in Xcode.
But it’s actually not that hard. Just open the XIB in your favorite text editor and change some values in the opening
<document ...> tag:
- set `type` to `com.apple.InterfaceBuilder3.CocoaTouch.iPad.XIB`
- set `targetRuntime` to `iOS.CocoaTouch.iPad`
This should do the trick.