If using GPS for altitude, how do I set the Baro as a back up?

ArduCopter Copter 4.1
1

Hi all, any ideas how to set the Baro for backup if using GPS for altitude? Many thanks.

2

EK3_SRC1_POSZ: Position Vertical Source as GPS and EK3_SRC2_POSZ: Position Vertical Source (Secondary) as Baro!