At the top right of the screen, in the gray banner, you will find a suitcase graphic and "My Disney Vacation." Put your cursor over that and it will show a drop down menu. At the bottom of the menu is Dining Reservations. Move your cursor down to that and click on it.
If you are not already logged in to the Disney site it will ask you to log in.
Once you are logged in you will need to repeat the first step (go to My Disney Vacation)
You will get a screen that reads Retrieve Dining Reservations.
Enter your confirmation number for the reservation you wish to cancel, and the phone number you used to book it. Click Retrieve Dining.
Scroll down to the info on the reservation. To the right there will be a button that says "Cancel." Click that.