Lemon has bleaching powder but you can increase its effect by adding few natural ingredients.
You can mix lemon juice with 1/2 cup of baking soda.
Apply it on affected area and rub it gently.
Leave it for few minutes and wash it off.
You can also add few drops of olive oil or any essential oil or honey.
You can mix 1 teaspoon of baking soda and lemon juice in 2 teaspoons of warm water.
Apply on the affected area regularly.
Note: You can replace the lemon juice with no pulp orange juice or grapefruit juice.