there can be mutiple possibilitities of the spots. But most commonly are due to sun tanning
, cutaneous amyloidosis
and Lichen planus pigmentosus
It would be better for you to consult a doctor for the best advise. You can also send a direct query with all the details regarding your age, any other illness and any medication intake along with a good resolution picture of the area to guide you best at the below a link
For the mean time you can apply a cream containing Kojic acid or glycolic acid
with best sun protection.