Hold the toothbrush with one hand on the handle just below where the brush attaches.
Locate the seam toward the bottom where the battery compartment screws into the handle.
Grasp the battery compartment and twist it counterclockwise to open it.
Remove the old battery, noting how it is positioned in regard to the positive and negative terminals. Dispose of the old battery.
Insert the new battery into the battery compartment the same way the old one was positioned. Reattach the battery compartment by twisting it clockwise to tighten it.