REGARDING GSTR-1 MISTAKE

Shoeb Hasan (81 Points)

22 August 2019  
Dear sir,
there is a mistake in gstr-1 qtr 3 of 2018-19
i have issued 3 invoices to x party but in gstr 1 due to human error, i have entered gstn of y party. and file my all returns till now.
now what options i have.
please tell
how can i cancel those invoices from y party and issue to x party. x party have claimed that itc.
how to correct it.
i want to know every possible way.
i think i cant issue credit note now. because i have filed all returns till now and also it has been 8 months from that period.
please help