Summarize the following:
If there are fingerprints or other stains on your phone's screen, use microfiber cloths to clean them. You can purchase microfiber cloths at most drug stores or department stores. The texture of these cloths can easily remove fingerprints and stains. Do not substitute the microfiber cloth with soft tissues like Kleenex. These can break during the cleaning process and stick onto the lens or scratch it. Take the microfiber cloth out of its package. Very gently swab the surface of your iPhone's camera lens. Wipe your lens down as long as necessary to remove unwanted stains and fingerprints. It is not necessary to use cleaning products on your iPhone's screen to remove stains. In fact, cleaning products could potentially damage the iPhone screen. Stick to dry microfiber cloths without added water or products to clean your iPhone.

Summary:
Opt for microfiber cloths. Wipe down the lens gently. Do not use chemical cleaning products on your iPhone screen.