What is the best way to reduce the swelling of my left labia minora (lip)?

Cold 20min off/on: If the swelling is due to recent trauma, ice is the best treatment to limit swelling and possibly reduce it. If it is unknown origin, you should have it evaluated.

Several ways: This is usually from intercourse but there are pathologic processes that can swell the labia. Cold compresses, ice packs & occasionally topical steroid should reduce the swelling. If persist, see a physician.