In the Financial year 2019-20 by mistake B2B sales was wrongly entered as B2C sales, Now how this mistake will be rectified , i have filled my all GST returns for the above Financial year. and in the current FY i have filled 3b upto Dec 2020 but GSTR 1 was still pending can i do any adjustment of the mistake of fy 2019-20 in the GSTR 1 of 2020-21

Reply— 

As per provisio to Sec 37(3) no rectification of error or omission in respect of the details furnished under sub-section (1) shall be allowed after furnishing of the return under section 39 for the month of September following the end of the financial year to which such details pertain, or furnishing of the relevant annual return, whichever is earlier.

Thus rectification for the year 2019-20 could have been done till Sep 2020 now you can not make any amendments.