Read this about how to change the security questions: http://support.apple.com/kb/HT5312
If you can't use these procedures contact Apple. See http://support.apple.com/kb/HT5699
You can also call your country number from http://support.apple.com/kb/HE57 and ask to speak with Account Security.
You can also go to the ExpressLane at https://expresslane.apple.com and then iTunes > iTunes Store > Password and Security Questions and fill out the form.