Gst return regarding

Shift from composition to regular can be done at any time after which you will have to comply with the regular GST provision let filing GSTR 1 2 3 3B

Which returns file for September he changed composition to regular in month October and how

Yes... He will file GSTR 4 for the period which is the dealer opt Composition periods...
After that he can file regular returns...

Ex :
The said dealer 1st sep to 20th sep under composition scheme and next from 21st he opt regular then he will file GSTR 4 return 1st sep to 20th sep on "July-Sep" GSTR 4 return and also file GSTR 3B and GSTR 1 returns...

Yes agreed with Raja sir

answer by raja p m is correct

Thanks for the corporation

Accept the view of Raja sir.
you can file returns as per table shown while selecting Financial year and month.
For eg. if it showing quarterly status , file gstr4 returns for that quarter. File 3B and Gstr1 for monthly shown periods.

generally if we chose one type of return filing then we can't opt for another in the same year