To view a plugin editor, double-click on the plugin within the processor box. A new window will appear showing the editor/GUI for the plugin.

If a plugin does not have its own GUI, Ardour will construct a generic plugin editor from a small set of common control elements. Ardour will do this even for plugins that have their own, if you disable Edit > Preferences > GUI > Use Plugins' own interface instead of Ardour's

You can temporarily switch to the generic UI by context-clicking on a processor and selecting Edit with generic controls. This will be necessary to access the plugin automation controls.

In the generic UI, you can re-set any controller to its default by Left-clicking on it.