To change the data you entered during registration, such as the start date of your first cycle, or the average length of your cycles, click on Settings (located in the upper right corner of the site), then in the "my account" tab find "initial data". You must change the initial data before your first cycle is complete. After your first cycle is complete, you will not be able to change the date of the start of your first cycle.
Be aware that, by changing the start date of your first cycle, you will lose all the information entered into the chart up to that point. (e.g. temperature, symptoms noted since the beginning of the cycle, or medication taken).