You can amend or cancel your flight plan after it has been filed with either the CANCEL or AMEND buttons in ForeFlight Mobile on the Flights page, by calling the 1-800-WXBRIEF phone number, or through the DUATS web site if you entered DUATS credentials under MORE > ACCOUNTS.
VFR plans can be amended (if not activated) or activated until 2 hours after your ETD. Once a VFR flight plan is filed, it can be amended (unless activated) or closed at any time using ForeFlight Mobile.
IFR flight plans can be amended or canceled until 30 minutes before the ETD, at which point the plan is transmitted to ATC.
Filed plans are automatically canceled if not opened/activated within 2 hours following the departure time.
All ICAO flight plans are automatically filed with Leidos. FAA/Domestic flight plans are filed with Leidos unless you have entered your DUATS credentials in the app under MORE > ACCOUNTS.
Flight plans filed using DUATS may be able to be amended through the DUATS web site, provided you used your DUATS account to file the plan. However, a plan that DUATS has already sent to the appropriate agency cannot be amended. From the DUATS website:
"Flight plans that have already been transmitted to the FAA cannot be canceled or modified. VFR flight plans are usually transmitted to the FAA 1 hour before planned departure time. IFR plans will typically be transmitted from 2 to 6 hours before planned departure time, depending on FAA requirements."