Note the model number of the garage door opener that is attached to the garage door. The model number typically will be located on a tag on the body of the motor. Compare the model number with the switch position diagram in the manual that comes with the Genie GT912-1BL.
Turn the Genie GT912-1BL over so that the back is facing you. Unscrew the back compartment with a Phillips jeweler's screwdriver. Remove the back panel from the Genie.
Insert a compatible battery into the battery slots inside the Genie -- positive (+) and negative (-) labeled text in the Genie will enable you to line up the battery contacts properly.
Change the positions of the switches inside the Genie so that they match that shown in the manual -- the tip of a flat-edged screwdriver will push each switch up or down so that it matches the manual's diagram.
Close the back panel on the Genie. Reattach the screw.