remove pills by hand, soak the scarf in water with hair conditioner (may help to revive the softness and brightness of the yarn), block with pins to dry, if it is made in lacy pattern. But honestly, for a best result I would re-knit it after washing the skeins of yarn.
To revive an old scarf that's pilled, start by gently removing the pills with a fabric shaver or a sweater stone—just be careful not to snag the stitches. Lay the scarf flat and give it a good wash according to the yarn type, using a gentle detergent. After washing, reshape it and let it air dry flat to avoid stretching. If the fabric looks dull, a light steam blocking can help refresh the texture and even out the stitches. These simple steps can make the scarf look cleaner, softer, and much closer to its original condition.