There is no way to roll back (as far as I know!), but when you hit TAB (or the button on the down right corner of the screen) you can open a screen with a dialog history.I skimmed the comments and could't find an answer, nor with search. Is there a back button for lines of text in unity like there is in renpy? Thanks, and again I apologize if this has been asked here I haven't seen it.
Hope this is what you are looking for!