The battery can be easily replaced with a soldering iron (ask Google if necessary). Just make sure that the new batteries are really well placed in the holder (not too much insulating tape etc. so that the mechanics and motor are not clamped too tightly to the housing and can vibrate well. These batteries have proven themselves (frequent charging): Varta Recharge Accu Phone (battery, 2pcs., AAA, 800mAh)

Commercially available AAA batteries are installed in the toothbrush. You have to unscrew the toothbrush with a little force and solder the batteries. There is information about this online. I did this once, but with limited success, because the toothbrush was no longer optimally adjusted mechanically. I then bought a new one.