Q: how do i chage my securyty quetions when i forget the anwser

You need to ask Apple to reset your security questions; this can be done by clicking here and picking a method, or if your country isn't listed, filling out and submitting this form.

They wouldn't be security questions if they could be bypassed without Apple verifying your identity.

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Forgotten Security Questions/Answers

You need to contact Apple by:

1 - Use the Express lane and start here:

https://expresslane.apple.com

then click More Products and Services>Apple ID>Other Apple ID Topics>Forgotten Apple ID security questions.

or

Apple - Support -form iTunes Store - Contact Us

2 - Call Apple in your country by getting the number from here:
http://support.apple.com/kb/HE57

or          

Apple ID: Contacting Apple for help with Apple ID account security

3 - Use your rescue email address if you set one up

Rescue email address and how to reset Apple ID security questions

For general  information see:

Apple ID: All about Apple ID security questions