File Explorer
From the Tools tab in the service overview, click on the "File Explorer" button.
First launch can take 1-2 minutes because File Explorer is installed on your service. Next launch are nearly instant.
This will open a pop up window, providing you with a temporary password and Access link.
Copy the temporary password and click on the Access link.
Authenticate with the credentials on the file explorer page:
Once logged in, you'll be able to browse, edit, download and upload the files on the server: