You don't...you simply do not do it. I'm tempted to curse at how badly you shouldnt overclock a laptop.
laptops have very poor cooling in most cases, and the cooling is JUST enough to cool it at its current speed. You will gain no noticable benefit from overclocking a laptop, especially a Dell one, as it is not geared specifically for gaming (the cooling will not be as good as say an alienware (also dell) or msi laptop.