On the MDE app, go to your home screen, scroll down until you find the FPP you want to change, click “my plans”, click the FPP you want to change, click “modify”, choose the members in the party you want to apply the changes to, and that will bring you to the page where you can choose a new time or search by your desired times. If there’s an earlier time that you want, click and and confirm by pressing “save changes”. If there’s no better time, then just close out of the modify screen and keep the time you have. You don’t have to (and shouldn’t) cancel the FPP in order to change the time or choose a new ride.
On a desktop, just go into MDE, navigate to your “My Plans” page. Scroll down to find the FPP you want to change. Click “Update” at the far right, and choose a new time or different ride. If there’s an earlier time, click it and confirm the change.