The Braun Oral B electric toothbrush runs on a rechargeable battery and allows for long-term use, unlike a toothbrush simply powered by a battery. The Oral B website suggests that instead of replacing the rechargeable battery, you should buy another toothbrush. But if you want to replace the battery without replacing the entire toothbrush, you will need to gain access through the bottom of the toothbrush. You should attempt this task only if you are confident in your abilities with a soldering iron.
Locate the two notches on the bottom of your toothbrush. Grip the notches with needle-nose pliers and twist to the left. This will unlock the bottom cover.
Grip the toothbrush at the top and pull it away from the bottom, revealing the internal components. Locate the two wires that attach the battery to the circuit board and place a piece of desoldering braid over the connection points. Touch the braid to the tip of a hot soldering iron and free the wires.
Remove the bottom piece of the internal components. Apply pressure to the two clips that hold it in place. Locate where the circuit board is attached to the tooth brush by solder and use the desoldering braid to desolder the circuit board from it's perch. The battery will be exposed.
Remove the battery by separating it from its contact points and replace it with a new one. Solder the internal components in reverse order by using the soldering iron and solder.
Braun Oral B suggests replacing the entire toothbrush instead of replacing the battery.