How to claim input credit for the month of october?

When u will login ur Return Dashboard and enter GSTR3B Portal, u will see an option of Eligible ITC in the right corner where you can fill he required details, the Input tax credit you received through your purchase. 

If its a case that u did'nt claimed the ITC u required to claim in your 3B return of october, then such addition can be made during filing of GSTR 2 return when the portal will get open.

If the GSTR 3B return of OCT is only submitted then it can be revised, but if it has been fully filed then it cannot be revised. the GSTR 2 portal is not open till now. Visit gst home page for the updated dates of filng return