If you go into your Personalized Study Plan on your My Studies page, you can update your exam dates under the heading, "Update your Personalized Study Plan." Click the button "Select Date" next to your start date and then your end date to select your new dates. Be sure then to click "Update Personalized Study Plan" at the bottom of the page after choosing your new dates.
If you still are having trouble, please let us know a little bit more about the specific issue that you are having, such as - is the calendar not popping up? Is it not allowing you to choose a date?