error: There is no SDK with the name or path 'iphoneos2.2.1' - after Xcode upgrade

I upgraded my Xcode to the Latest version today, my iPhoneSDK OS was 2.2.1 previously. I was alerted during installation I can upgrade Xcode but iPhoneSDK cannot be upgraded. Because my Mac OS was 10.5.4. My iPod Touch runs on version 3.0.

After Xcode upgrade, when I launched old projects, it says error: There is no SDK with the name or path 'iphoneos2.2.1'. Under project settings the o开发者_如何学编程nly 'baseSDK' is 2.2.1. Also the frameworks are missing.

Whats the solution, I want to run the applications. An OS update would resolve this problem? Is there any way to manually configure the path to iPhone SDK OS in Xcode?

Thanks

If I change the SDK setting to iPhone Device 3.0 it works.