我改变了部署目标
以及如何更改基础SDK？或许我不能改变它？
解决方法
First you should understand a few things. Deployment target is the minimum OS version that your app supports. If you set into iOS 5.0 you can assure that it’s support iOS 5.0 and above. Base SDK is the SDK which is used to compile your application. Always use latest SDK available.
If you want to create app that support iOS5.0 and above set deployment target as 5.0. Then use latest SDK available. The reason is that if you change base SDK to some lower version,your app may not support ios6 since some of the methods are deprecated in iOS6. If you compile it with latest SDK it shows warning. You can test your app by installing different simulators to check wether it is working properly. Mainly you need to handle orientation issues since it is different in both.