On your computer or safari on your iOS device navigate to:
For Canada - https://appleid.apple.com/ca.
For United States - https://appleid.apple.com/us/.
Click the "Manage your Apple ID" button, and sign in to your iCloud account. Once in, on the left side select "Password and Security", and this will populate our options on the right side. You should see an option to reset your security questions.
Also see this article about your "rescue email address" - http://support.apple.com/kb/HT5312.
If for any reason you can not reset these security questions, or get an error, then you may need to contact Apple directly - http://support.apple.com/kb/HE57. Explain the issue, and they can get your to a "Account Security" agent, who will be able to verify your identity, and assist in resetting the security questions.
See Kappy's great User Tips.
See my User Tip for some help: Some Solutions for Resetting Forgotten Security Questions: Apple Support Communities https://discussions.apple.com/docs/DOC-4551
Send Apple an email request for help at: Apple - Support - iTunes Store - Contact Us http://www.apple.com/emea/support/itunes/contact.html
Call Apple Support in your country: Customer Service: Contacting Apple for support and service http://support.apple.com/kb/HE57