If you want to completely reset your Marvin to a clean state, including deleting all your tasks, open ☰→Account→Account, then scroll down to the bottom:
If you just want to clear all your settings and keep your tasks, then delete your workflow.
Doing a factory reset creates a new sync database for you and deletes your old database. It isn't possible to recover your old database from our servers, but you would be able to restore data using a backup. If you use the desktop app, then you likely already have automatic backups on your computer.