In the Displays preference pane, hold down the option key and click the "Scaled" radio button. This will toggle on and off additional resolutions for the device (including more scaled resolutions for the built-in display in MacBooks).
I'm not aware of any radio button ever having worked that way before, so it's easy to miss.
If your external display isn't showing an image at all because the OS got its "native" resolution wrong, you will of course have to click the "Gather Windows" button at the bottom of the preference pane to bring the window for the blanked-out display over to the working one, where you can then option-click it and select a good resolution.