Set the step ladder under your smoke alarm. Climb up and grip the cover of the alarm, turning it one-half turn counterclockwise and releasing it from its bracket.
Pull the cover off to expose the battery compartment. Take out the old AA batteries and insert two new ones, making sure that the positive and negative terminals on each battery line up with the matching terminals in the smoke detector.
Place the smoke alarm cover back into the bracket and turn it one-half turn clockwise to lock it into place.
Push the battery "test" button on the smoke alarm cover. Listen for the alarm and check to make sure that the light glows, indicating that the batteries are working properly.