Till September 2020 you can reverse in GSTR 3B , but after month September need to Reverse any such ITC need to furnish DRC 03 .
Note : In my view best approach is to file DRC 03 . because if you do Reversal in 3B of Fy 2020-21 , you need to prepare Reconciliation statement for FY 2020-21 also when you file Annual return for Fy 2020-21