typical bar magnets are 2.5"x0.5"x0.125"
the plane of magnetism is across the 0.5" dimension. north on one side, south on the other. gently pull the existing magnet half way out. and the south side of the new magnet will stick to the north side. note which is which, put it aside, gently remove the old magnet, then gently slide the new one in making sure to use the correct orientation.
you need to be a little careful so you dont break any wiring connections, but with care, its not a hard process that anyone should be able to do