You can't. Sunburn will go away when it wants to go away, and typically it takes its sweet time. Sit in a cool bathtub or apply cool wet dressings, and use a good anti-burn cream like silvadene (silver sulfadiazine) or bionect. And learn the lesson: mr. Sun is our friend, but we have to respect his power. Get your self a good sunscreen lotion (spf ~50) and apply it every day, whether you plan to go out in the sun or not.