@Daniqm I see it under Fitbit Settings.
Click to the Tile Menstrual Health, on the upper right, click the 3 vertical dots, Menstrual health settings - go to Predictions Set up values.
or
Click your profile icon (upper right) - Fitbit settings - Under Preferences go to Menstrual health - Under Predictions go to Set up values.
You should then be able to edit your period and cycle length.
Best Answer