short answer: They can't. It's a scam.
long answer: There's this thing called a placebo affect. Basically, if you believe something will make you feel better, then there's a good chance it will make you feel better. It's not that the item actually did something to you to make you feel better, it's your perception of the item that made you feel better.
Magnets "detoxing" the body is something that sounds lovely, but isn't actually based on reality. All the actual research points to magnets not influencing the body in any way. The claim is that magnets somehow affect the iron in your blood, or affect the "field" around your body. This is all made up. If you don't think about it too hard, it might make a sort of sense. but it's not real or true.
Basically, anything that states magnets will heal you in any way shape or form, is a scam.