On you computer 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 - 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 at 1 800 692 7753. 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.