Hi it is normal for constantly sun exposed arras such as arms , face and neck to get tanned. Regular use of a sunscreen as well as protective clothing prevent further tanning and over a period of time can help lighten the pigmentation. you can try chemical peels and laser for skin lightening however I think as the condition can recurring very easily with exposure to sun I will recommend that you consider waiting for it to resolve naturally. this may take a few months. please contact a dermatologist who can prescribe oral vitamins and exfoliation creams to help you