At the top of the site, click on your name once. A menu will appear, then click "Account details". On the new page, if you scroll a bit further, you'll see fields for your date of birth, where you can change the month, day, and year of your birth.
Just curious, when do you go back to school in Canada? Where I live in America summer break ends in mid-August, I was just thinking about what it's like in different countries.