you need to get the selected month from your combo to a integer value and set the datetime picker value based on that, for example if you select 3rd month
int monthvalue =3;
dateTimePicker1.Value = new Date(dateTimePicker1.Value.Year, monthvalue, dateTimePicker1.Value.Day);
if you need to change the datetimepicker date, when you select month from from combobox, then you can place above code in the combobox selectedIndexChanged event.
when you bind the month data to combobox, set value member as integer and display member as month name, then you can parse the selected value of combobox to a integer value and set it as datetime picker month as above.