I have my own account in itune and app store with my ipad. Recently my son purchased some items using my ipad via his own itune account. Since then, whenever I tried to update Ibook app, it asked my son's passwards showning my son's account. My son never tell me his passwards. I checked the setting, and the app store account is done with mine not my son's. How could I fix the problem? please help me.