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How to sign out of App Store
If you are having difficulties trying to purchase in-app upgrades, there may be a problem with your Apple ID on your device. The first method of rectifying this, is to simply try signing out of the App Store on your device, reboothing the device (power off and power back on), then sign back into the iTunes App Store with your Apple ID and credentials.
Here is how to perform that task: http://youtu.be/jn1VCi5mBWo