You can't change them if you forgot the original answers. You can contact itunes store support and they will guide you on how to call the department who can reset them:
Follow these steps:
All Products and Services > iTunes > iTunes Store > Account Management > iTunes Store account security
Answer the questions and if required sign in with your Apple ID and password - same one you used to log in on this support forum.
You will find an option to send an email to Apple on that page. Indicate that you forgot the answers to your security questions and wait for iTunes support to reply. Responses usually take 24 hours.
Usually you will need to contact iTunes store support but while waiting, you might wnat to try this out:
-On the device/computer that is asking you for the security questions go to "settings", > "store" > tap the Apple ID and choose view"Apple ID" and sign in.
- Tap on payment information and add a credit/debit card of your preference then select "done", in the upper right corner
- Sign out and back into iTunes on the device by going to "settings"> "store" > tap the Apple ID and choose "sign-out" > Tap "sign -in" > "use existing Apple ID" and you should be asked to verify your security code for the credit/debit card and NOT the security questions.
- At this time you can remove the card by going back in to edit the payment info and selecting "none" as the card type then saving the changes by selecting "done". You should now be able to use your iTunes store credit without answering the security questions.