How to get current month wise get last month data

Question

I want get current selected month dropdown(september) this month get previous month data in Asp.net with C# or sql server

Below code I am using but result was not came

Sql Query:

select substring(DATENAME(MONTH, ProcessDate),1,3) as Month from Accounts_DailyAttendance where datename(MONTH,ProcessDate)='September'

The above query wise

i want result is October month data

Thanks