Start here(change country if necessary)and navigate to 'Password and Security', reset your security questions using the link provided, you will receive an email to your rescue address, use the link in the email and reset your security questions.
If that doesn't help, you don't receive a reset email or you don't have a rescue address, you should contact AppleCare who will initially try to assist you with a reset email or if unsuccessful will pass you to the security team to reset your security questions for you.
If you are in a region that doesn't have international telephone support try contacting Apple through iTunes Store Support.
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