gstr-1 and sales return

Dear all
we have some sales return received the goods through credit note.

credit note figure put in 3b with out gstr-1

hence mismatch in 3b and gstr-1

some cases was in year of 17-18 and
some are year of 18-19

now what is the corrective action to do for 17-18 and what is for 18-19 ?

need your expert opinion please.