You would create another group or poupup, with input elements of those data you want to edit.
Make sure the popups source type is of the same type of the repeating group,
Then set each input element’s initial data to parents thing’s email for example.
Then add an icon on the repeating group with a workflow that will show this popup and “display current cells thing”.
On the popup add a save button with a workflow to “make changes to thing” to parents thing.
Another action that will hide the popup.
Its also good to add a cancel button with a workflow to hide without saving.