You can set or change your Recovey Questions for your Runescape account in a Jagex-related Website.
Start at the Runescape Homepage, which is www.runescape.com. See those multiple tabs under the banner? Hover the mouse over the Account tab. See Account Management? Click on that. Look around - you should see a section dedicated to Recovery Questions - choose the appropriate link. You will be asked to type in your Runescape Username and Password - do that. There, you will have the option to set your Recovery Questions and Answers.