Lemon juice and baking soda, mix the lemon juice and baking soda then apply and leave for 1 minute.
But dont over do it (as it can weaken our enamel) especially if you have sensitive teeth.
Better to avoid or lessen your consume of foods that can cause yellow stains
One important thing we need to avoid to get free from stains is coffee which is why most people got yellowish black teeth and also smoking and spicy foods.