Hello Any body knows how remagnetize the magnets in ad DC motor. we're removing the magnets and then a supplier from us is remagnetizing the magnets, and then we put the magnets again. But my question is how this supplier how is doing that job.

Had done that before:-- need to de-magnetize & than magnetize. But for your case -- before doing that, what's the reason as I interpret you put the magnets in magnetized condition? (Since after you remove the magnets from the motor, the magnets are in magnetized condition. Of course I also presumed you have valid reason to remove the magnets at the first place -- eg. damaged Rotor but recover the magnets.)