If the ITC pertains to 17-18 and not declared in sep'18 you can claim up to Mar'19 through GSTR-3B. Thereafter you are not eligible to avail 17-18 credit. The ITC which is not claimed has lapsed no need to declare in returns.
Reversal of ITC you can reverse by using DRC 03 and declare the same in Table 12 of GSTR-9