Q: how can I change my security quedtion

From http://support.apple.com/kb/HT5665 :

• If you have three security questions and a rescue email address
  • sign in to My Apple ID and select the Password and Security tab to send an email to your rescue email address to reset your security questions and answers (the steps half-way down that page should give you a reset link)
• If you have one security question and you know your Apple ID password
  • sign in to My Apple ID and select the Password and Security tab to reset your security question.
• If you have one security question, but don't remember your Apple ID password
  • contact Apple Support for assistance. Learn more about creating a temporary support PIN to help Apple confirm your identity when you contact Apple Support.

If you can’t reset them via the above instructions (you won't be able to add a rescue email address until you can answer your questions) then you will need to contact iTunes Support / Apple in your country to get the questions reset.

Contacting Apple about account security : http://support.apple.com/kb/HT5699

When they've been reset (and if you don't already have a rescue email address) you can then use the steps half-way down this page to add a rescue email address for potential future use : http://support.apple.com/kb/HT5312