You can edit your past periods at anytime by clicking the Period button on the upper righthand corner of your home page (the calendar icon if you're signed up on Android) or by clicking your first Daily Gem and then clicking "Calendar View" at the center of your cycle visual. Simply tap on a red period to edit or delete it or tap on other days to add new cycles. To edit a period, tap on it and then drag the blue circles around until the start and end dates are correct and click 'Save' in the upper right. To delete a period, tap on it and then click the 'Delete' button in the upper left on iOS or the Trash icon on the upper right on Android. To edit your cycle length in Eve, head to your Me page (the person icon on Android), click 'Health Profile', and then scroll to the cycle length and click on it to edit. However, please note that this value is only used to predict your first cycle. After that, Eve will predict your periods for you based on your past periods.