Q: How do I change my security questions?

If you've forgotten their answers and you have a rescue email address (which is not the same thing as an alternate email address) set up on your account then the steps half-way down this page should let you reset them : http://support.apple.com/kb/HT5312

If you don't have a rescue email address (you won't be able to add one until you can answer 2 of your questions) then you will need to contact iTunes Support / Apple 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 the HT5312 link above to add a rescue email address for potential future use