To access the configuration screen of a fieldset simply select it in the tree structure in the Form Template configuration screen.
You can define the name of the Fieldset and the status.
Inside the Fieldset there are the fields visible inside the Form.
On the right side there are all the available fields, in the center those visible inside the selected Fieldset.
You can show a field in a Fieldset by simply drag and drop it from the Available Fields to the Fieldset.
To delete a field from a Fieldset simply drag&drop it from the Fieldset to the Available Fields area on the right. At that point, the Field will be available to be inserted in other Fieldsets.
Note: fields cannot be replicated on the various Fieldsets, so if the field is inserted in a specific Fieldset it cannot be replicated also in another Fieldset (for example) in another Tab.