Press [Dup./Combine/Series].
Make sure that [Duplex] is selected. If [Duplex] is not selected, press [Duplex].
Press [Orientation].
Select [Top to Top] or [Top to Bottom] for [Original:] if the original is two-sided.
Select [Top to Top] or [Top to Bottom] for [Copy:].
Press [OK].
The default is [Top to Top]. You can change the default orientation under [Original Orientation in Duplex Mode] and [Copy Orientation in Duplex Mode] in User Tools. For details about these settings, see General Features.