Command-Option-Shift-K to clean out the build folder. Even better, quit Xcode and clean out ~/Library/Developer/Xcode/DerivedData manually. Remove all its contents because there's a bug where Xcode will run an old version of your project that's in there somewhere. [...]

In the simulator, choose iOS Simulator > Reset Content and Settings.

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caching - How to Empty Caches and Clean All Targets Xcode 4 - Stack Overflow.