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 to 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.
Deleting Habits, Goals, Labels, etc.
There isn't an easy way to delete these so you'll need to delete them manually. To delete Habits and Goals, you'll have to open their views by pressing "H" or "G", respectively. For habits, you can either delete a habit when you click on it, or just delete its history by choosing "Edit History". And to delete a goal, open it, click on the "Settings" icon on the top-right and in the Settings you will see "Delete"option.
To delete labels, go to ☰ → Strategies → Labels (settings) and click on "Delete all" button.