Gst charge wrongly

Hello, 

if trader generate wrongly sales bill .. query is if Trader generate sales bill by charging wrong cgst and sgst and file gstr3b what should do now.

in detail ...

Trader buy goods at 28% gst from manufacturer , now due to software mistake he sale that product at 12% (6cgst+6 sgst) after one month he notice that he is making wrong sales bill , he has already  filled gstr3b .. now what to do ,? as if there is no provision to revise gstr3b .

can someone advice on this topic.