If you've mistakenly shown IGST instead of CGST/SGST in your GSTR 3B for February 2021, you cannot directly edit the filed return. You can use Form PMT-09 to reallocate the tax if it hasn't been utilised. Alternatively, you can correct this in the following month's GSTR 3B by paying the tax under the correct heads and claiming a refund for the amount paid under the wrong head. No interest will be charged for this type of correction.
24 April 2021
Use PMT-09 to reallocate the tax, if not utilized. Else you need to rectify in the next GSTR3B by paying tax under right heads and applying for refund under the wrong head. No interest would be applicable.