How to downgrade iOS from 9.3.1 to 7.1 (default version)?
You can't install older versions of iOS (see Can I install any iOS version I wish by picking and choosing which firmware to restore? for details), but you can run the simulator in Xcode with older versions.


From within an iPad project, go to Product -- Destination -- Download Simulators...
Pick the version you want


It doesn't go further back than iOS 8.1 though.