From the extension's URL (what you would send to your end users), click Edit Extension in the bottom left corner. This will open up the configuration in a modal (popup) so you can make adjustments, as needed. If you first open up a child form (by clicking on a linked record), the button will read Edit Child Extension, and open up that child form's configuration when clicked. This is a great way of navigating to exactly where you need to be in the configuration!

Find the extension on your miniExtensions dashboard, by navigating to the correct base and then selecting the extension.