[Mileage712]

Hi guys, I have a Magnet Marshal but long time ago lost the manual.
Now I need to replace the battery but I don't know how.
Does anyone have instruction for me? Or maybe a Pdf from the manual?

I would really appreciate some help, don't want to destroy the device 

Depends on the unit. The original required removing the screws from the botton and you'll see a cr2032 battery. Once the back is off look close at the very narrow metal spring clips that hold the battery in. Move the spring clip over, remove the old battery and insert the new battery. After that put the screws back and that's it.