At the top right of your main dashboard page, click on the gear icon. On the resulting window, click the My Preference button to view and edit your user preferences.

Before you can set your security questions, you will need to scroll down to the Password section and enter your current password in the Current Password box. After entering your password, simply click outside of the text box and, if the password was correct, the Change Security Questions section will be enabled

Select your preferred questions and enter the answers, then click the blue save button at the bottom right of the window.