Disclaimer! The addition of a username and password prompt should not be treated as a failsafe security measure. It does not guarantee limited access to the underlying data (for more secure data management, see Using Private Datasets). Think of it as a slipcover over a nice car, not the keys.
Locating the Password Prompt
From the landing page of the admin panel (admin.finance.socrata.com), navigate to the site-specific page for the instance you are trying to update. From here, go to the "Other Settings" page through the navigation options on the left hand side of the page.
Note: Though this image reflects the Open Expenditures Admin panel, all SPF products have this tab in their admin panel.
From here, use the text box to set a password for the site (or remove the listed password) and then save your changes.
The value you have set will act as both the username and password in the prompt on the site. You will now be prompted for a username and password when you move to access this particular instance of your finance app.