You can't cancel epics once you start them, but you can decide to leave the fleet.

If you wish to leave a fleet, please go to Walkr > Menu > Epics > Members. Tap the "i" next to your name on the list. A Leave Fleet button will appear. Tapping it will allow you to leave the fleet! Note that if you leave the fleet, any contributed resources will not be returned to you. Please consider this carefully before deciding to leave any fleet!

If you're the fleet captain and decide to leave the fleet, the system will automatically appoint a new captain by selecting the fleet member who has contributed the most to the fleet up until that point!