If the toothbrush has some slots in the bottom that will accept a large flat blade screwdriver, then clamp the screwdriver in a vise, engage the slots on the screwdrive and turn the case until you hear a snap. Then the internals can be pried out easily using a screwdriver. If no slot, use the excellent alternative opening procedure.

Remove rubber ckt board cover & pull off the white cap at far left. Then unsolder wires at 6 points shown. While applying heat, extract solder with solder pump & solder wick. Gradually pry ckt board off the soldered points while applying heat. Then pull out the battery from the other side.

Detach motor assembly from battery holder. Cut through the spot welds (A Dremel tool and abrasive disk is what I used) and open the clamshell holding the motor assembly. Pull out motor, remove magnet attached to motor stator. This concludes teardown process.