Dennis @ PerfectForms
Because I don’t know your process, I can only answer your narrow question. For more detailed advice we would have to discuss your form in more depth.
You could use a set table rows object to move to the previous row. Then use a set field object to place the value(s) in that row into a helper field. Finally use a set table rows object to move to the last row and a set field object to set the value in that row using the data in your helper field.