Table 12 of GSTR 1

Yes , mandatory, please refer notification 78/2020 ( Central Tax) dated 15/10/2020  and confirm 

Yes, I agreed the REPLY...

Thank you for the reply and is greatly acknowledged.

I have not filled the table 12 details for previous periods. what shall I do with respect to previous monthly returns filed as updation is not possible...

Well...

As per Your query,
No way or updation in this missed details.

But,
Prepare and Keep Your details as per Your transactions with HSN details and Quantity...

In case Dept raise any questions or clarifications then give detailed reply and the mistakes.
May be dept can satisfied then the details may be supported.