From the website you can click the edit/cancel link by your reservation on the dashboard or find your reservation on the schedule and click it. This will start the Reservation Update Wizard. Make any updates you want, moving through the wizard with the Next button and finish with the Submit button on the last screen.

From the mobile app, you cannot directly edit a reservation, only cancel it. So you can follow the instructions above for the website or cancel the reservation from cancel link on the mobile app dashboard and then create a new reservation with the app. The edit feature will be added to the app in the near future.