ForeFlight follows the AIRAC 28-day cycle schedules for issuing data updates. New data always becomes active on a Thursday morning at 0901 Zulu. You will be able to download these updates inside ForeFlight Mobile 4 days ahead of time - on the Sunday before the cycle changeover date.
If you download data early, it does not display inside the app until it becomes active. The first time you open ForeFlight Mobile after the new data becomes active, the new data starts being used and then the old data is automatically deleted from your iPad or iPhone. You don't have to perform any additional steps.