Locate and carefully remove the small, see-through plastic "bar" from beneath the PIR's window	Put the "bar" aside, and now you will see a small philips screw
Undo the screw, taking care not to lose the small rubber washer	Carefully unclip the battery cover, and remove it to reveal the battery compartment and test/program buttons
Fit the batteries and test the unit as per the instructions. Reverse the process to re-fit the battery door.