If you want to completely reset your Marvin to a clean state, including deleting all your tasks, open ☰ → Account:

Then open the "Trask & Delete" tab, and there you will find the "Factory Reset" button:

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.