wrong charge gst rate on sale bill

Nitesh agrawal (student) (63 Points)

28 May 2018  
As per the old gst rate schedule Rate on medical oxygen is 18%
But after 10th nov it has been revised to 12%.
And i m charged 18% on bill upto april 2018 n filied GSTR 3B UPTO April 2018.

How it is rectified? Plz suggest.