Simply click the + icon and you will be able add or remove a period by selecting and de-selecting your flow.
If you are adding a period, by tapping on a particular flow, you will see immediately at the top of the screen that the selected date is now encircled in a solid pink bar. Click the checkmark (Android) or select Done (iPhone) to confirm your period.
If you entered a wrong period in the past, click the + icon to reach the edit period page, and you can then scroll backwards through the dates at the top of the screen, choose the date you want to remove a period, and de-select the flow to remove the past periods accordingly. Remember to click the checkmark (Android) or select Done (iPhone) to confirm your changes.
Please note that you cannot change future period dates. Once you land on a predicted period date and your period has not started yet, you can click the + icon, de-select the flow for today, and the start date of your predicted period will be pushed back to tomorrow.