Use lemon juice. Lemons are an age-old folk remedy for bleaching the skin. In the article “Lighten Dark Underarms Naturally with Lemon” on the Hosia Online Web site, the author writes that lemons are a natural bleaching agent. Apply lemon juice to the armpits with a cotton ball. You can also put the lemon juice in a small spray bottle. Use the lemon juice a few times a day for faster bleaching.