How do I delete a database from Firebase?

THIS DELETES THE ENTIRE PROJECT

To delete ONLY the database just delete the top level object/node.

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Click on your project (the white box below, hiding name):

Then click on gear (settings) icon and click "Project Settings":

Scroll to bottom and click "DELETE PROJECT":

Another way if you don't want to delete the project, and just the database, is to create a file on your computer empty.json like this:

{}

Then click on the ... and "Import JSON"

Then "Browse" and upload your empty file:

You overwrite your database with an empty JSON file, effectively "deleting" the old database.