I want to change the base SDK on xcode 4.2, how can I do this?
You have to give Base SDK as iOS 5.0, deployment target as OS version that you want to run (4.2?).
Most important thing is you can't use any API's/ libraries introduced in iOS 5.0. If you want to use them, make sure they are only called if running OS responds to them..If you are calling any methods which is not present in iOS 4.2, you have to check first whether they are available on running OS..This question might help you there..This blog post might also helps..