There are two ways you can access your extension's configuration:
For Portals and Interfaces: From the extension's URL (what you would send to your end users), click Edit Extension in the admin toolbar at the top. This will open up the configuration in a modal (popup) so you can make adjustments, as needed. You can also navigate back to the miniExtensions dashboard this way.
If you open a child form (by clicking on a linked record), there will be an Edit Form button in the child form's header, which would open that child form's configuration. This is a great way of navigating to exactly where you need to be in the configuration!
Some parts of the extension will also show a gear icon when hovering over it, which then takes you straight to the relevant configuration page in a popup (modal).
For Forms: From the form URL (what you would send to your end users), click Back to Settings in the admin toolbar at the top. This will open up the form builder where you can configure the form, as needed.
Find the extension on your miniExtensions dashboard, by navigating to the correct base and then selecting the extension.
Forms open in the form builder by default. If you prefer or need the previous editor, select the gear icon in the form builder toolbar to open form settings, then use the editor-mode control to switch to Classic.