Don't do it. Even if you can. Laptops are not designed to cope with the extra heat generated by overclocking. Most are on the edge of what you would call acceptable and some are poorly designed from the get-go, suffering random shut-downs caused by heat.
There are one or two laptop maufacturers who have included an 'automatic' o/c function that provides something like a 15% boost but they are built with cooling that can cope. (Just about).