Lay the Tennis Tutor on its side in the typical functioning position.
Use the screwdriver to remove the six screws holding the two large pieces of plastic casing together.
Remove the additional three screws holding the top half of the plastic casing together with the metal framework.
Take out the metal framework and the wheel right next to it. Use the screwdriver to remove the screws from the exposed switch plate.
Use your hands and pop the top half of the case off.
Unscrew the screws attached to the metal strap protecting the existing batteries and remove the old batteries.
Install the new batteries and secure the metal strap to keep the new batteries fastened and protected.
Reassemble the other parts of the Tennis Tutor in the reverse order that you removed them.