There are two ways to delete all your tasks and projects. With the first way, you'll delete categories too and with the second way, you'll only delete tasks and projects.
Using Master List do delete everything
Once in the Master List, press shift and click two categories to select them and every category in between them. You can also press cmd/ctrl (Mac and Windows respectively) to click on individual categories to add them to your selection.
Once you've selected everything, or just categories you want to delete, by clicking on "Delete" button you will delete all selected items. Keep in mind you can't undo this so it might be wise to backup your data before deleting it.
Using Search to delete all tasks and projects
To open the search dialog, press Shift + S on your keyboard.
If you are in the day view you can also click the magnifying glass icon at the top right.
At the bottom of Search window you will find two options: "select all tasks" and "select all projects". Using these two options you can select all tasks/projects and delete them by clicking on "Delete" button that will appear after selecting items.
If you also want to reset your Marvin to default setup read this article.