INTERSTATE SUPPLY SHOWN AS INTRASTATE

We showed by mistake an interstate transaction as intrastate in July 18 GSTR-3B and paid CGST and SGST instead of IGST.

Can any one suggest me how to get the CGST and SGST refund to pay IGST ?

In refunds section an option is there that interstate supply considered intrastate and viceversa. But it displaying a message that your sep 18 3B return is not filed , if try to apply.