Rectification in september return

Not understand your query property

A sale bill wrongly entered in jan 2020 and sale is not done but that bill is entered in gstr3b.now the amount of that sale and tax is more than sale of september mth.so my query is whether it can be reduced from sale and tax of sep and august as well....as sale and tax on it is entered of a wrong bill in gstr3b of jan 2020

You can not rectify of PY transactions after September month GSTR 3B due date

You can rectify in Annual return as per my opinion

How you file gstr 1 of that period

In Jan 2020 have you paid the tax. if so then adjust in gstr 9 for 2019-20 ay.

Gstr1 is ok there is no mistake in r1 and tax is paid in gstr3b with excess amount