If it?s on the outside of the camera lens, use a can of compressed air and a microfiber cloth. If it?s internal, you cannot without dismantling the phone/camera. In that case, take it to an Apple Genius Bar and let them look at it. Depending on the model and how long you?ve had it, it could be covered and replaced under warranty.
BTW, the Apple warranty, the Genius Bar and their customer service are just a few of the many reasons that I buy and use Apple products.