If your belly button stinks it could be because you haven't washed out your belly button like you should at least once per week using some soap or rubbing alcohol.
Dirt, sweat and bacteria and germs can hide in your belly button even if you cannot see it and then cause a smell to come from your belly button.
So cleaning your belly button can sometimes help get rid of the belly button stink smell.
If that's not it then the belly button can also stink due to fungal infections in your belly button or the fungal infection could be Candida that occurs on the skin.
In that case you can use some fungal cream to help kill off the fungal infection or use some Apple Cider Vinegar to help kill off the fungus.