Upgrading a Target from iPhone to iPad

If you have an existing iPhone application that you want to upgrade to run on iPad devices you need to upgrade the target that builds your iPhone application into a target that can build both an iPhone and an iPad application, or add a target to your project for building the iPad application.

To upgrade an iPhone target for iPad development, select the target in the Groups & Files list and choose Project > Upgrade Current Target for iPad.

For details about iPad-application development, see iPad Programming Guide.

ref: http://developer.apple.com/iphone/library/documentation/Xcode/Conceptual/iphone_development/115-Configuring_Applications/configuring_applications.html