You cant increase what its labeled as but, the rating is based on, Axle, Brakes, Trans, Frame, Suspension. You can increase how much weight it can hold but, can you stop it, are you going to burn up the trans and how well will the frame support it.
One of my old tow rigs had an 11k capacity. The frame and rear diff was the same to 14k. I upgraded the trans, added in some new springs, a couple of coolers, front rotors and was done. It was still an 11k vehicle but had no issue hauling 14k.