Reduction of Taxable value is reported in 3B

1)At the time of filling of gstr-3b for the month of sep-18 we inadvertently taken the taxable value and paid the tax amount but actually in books the taxable supply was lesser than supply shown in 3B so that i am rectifying in sep-19 GSTR-3B return (I am reducing the excess value what we taken in sep-18 ) if its posible. and also regarding the gstr-9 where i have to shown the difference?

2) continuation of (Q1 )if its possible i have to shown in gstr-9 for the period 18-19 in table 11 its correct or not