Demagnetize it, then remagnetize it the opposite direction. It's a lot of work and it would undoubtedly be easier just to repaint it and claim you reversed the polarity.
Well, the easiest way to reverse the polarity of a magnet is to just turn it around. However, inside of every piece of steel, there are magnetized pieces that will make the steel an induced magnet...
Since magnetite is already magnetised when found in nature, it is called natural magnet. Eg: Lodestone Magnets that are made by man are called artificial magnets.
Degauss it and remagnetize it in a field. Best when heated over the magnets Currie point. Cooling it down in a field the reverse of what you started with.
Natural Magnet
The lodestone is a species of iron ore which has the property of attracting iron
and some of its ores. When freely suspended it always points to the poles.