If you file a USA Domestic flight plan and have set up a DUATS account log-in within ForeFlight Mobile under MORE > ACCOUNTS, then filing will be with DUATS. You will be able to use your DUATS account using www.DUATS.com on the internet (not ForeFlight Mobile) to amend or cancel your flight plan. This can be done up to two hours before departure for an IFR flight plan.
If you don't have a www.DUATS.com account setup within ForeFlight Mobile, then ForeFlight Mobile will use Leidos for filing. You can use the "Cancel" and "Amend" buttons within ForeFlight Mobile (File & Brief page), to cancel or amend your flight plan. This can be done up to 30 minutes prior to your filed departure time.
You can also use 1-800-WX-BRIEF to cancel and amend flight plans.