This is done from the Notifications Preferences section of your Profile.
Go to your web Profile page. Click the Edit profile button. Locate the Notifications Preferences section near the bottom of the page. Adjust the value of the Upcoming Workout Email field to your desired time. Note that times after 17:00 (5pm) represent the previous day, i.e. the email is sent the day before the workout instead of the day of the workout.